So I was asked to GM a group of players that was being hosted at a meadary. I get free mead to be the GM so I thought what the hell. The problem was the game consists of 8 level 4 players, including one fool that snuck an unearthed arcana Ranger under my nose. I tried to give them a challenge by throwing them into a dungeon that was meant to be the third game mission but they walked through that and beat the boss like a misbehaving 5 year old at wall mart. I tried the x2 enemy trick but that doesn't do much to help. Any ideas on how to deal with a large party? Should I throw a nastier monster at them risking it overpowering the players, or should I just double the HP of the Mob?

I'm a first time dm with a group of 7 dealing with the same problem. I'm afraid of overcorrecting and killing them all. Also tracking initiative is a pain and the combat rounds take foreeever. Wondering if the solution is to contrive ways to make them split up.

I'd say toss some traps in on top of the usual problems. Something that blasts fire at the party once a round or dangerous points on the ground. Then either they have to spend time disabling the traps, or take extra damage every round. Gives some of the specialized characters something to do so they're not all just dogpiling enemies one by one and turning combat into a cakewalk.

I would have to ask first if you are just using creatures from the DnD 5e books? Most of the current monsters in 5e are very good for early encounters for smaller groups, but they tend to fall off fast when the party gets stronger and with better gear and spells. What I am about to suggest are ways to beef up or change the enemies of your encounters to better suit your needs and to give some umph where its needed. 

1. Action Economy- Sometimes a extra action can really change how intense a fight can be, especially for those enemies you intend to be above average or boss level. 

2. AC, HP, Resistances, and Immunity- The most common problem I normally encountered is that the party just kills the enemies to fast so I can only imagine 8 players on 1 boss scenario. In those cases a bit of extra hp and a few points of AC might bring out a bit more value out of the monster, and resistances are good for certain themed monsters or those with enchanted armor. Seriously half damaging can make a mini boss look a bit more big and bad than it should be. Immunity is also something that can be used under right circumstances and should not be used very liberally. 

3. Player levels, feats, and gear for humanoid enemies- For later on or a campaign where you want to use kobolds or goblins as your main enemy these can be very helpful in adding a bit more to them as a goblin with 2 levels in fighter gets so much more than before. Also enemies of rank or bosses a magic item or feats can make things a bit more interesting.

4. Lair Actions- People seem to forget about these as many monsters in the monster manuel have these. Basically they are simple action the monster can use if in their lair resource free. Great for bosses. 

This all I can think of so far, and there are probably better solutions than mine, but its always worth a input. With these I would suggest asking the question "what kind of enemy do I want?" I would say ti be safe that it has to do on what do you want accomplished. Want annoying minions to aid a boss, make them 1 hit to kill monsters with low ac but normal attacks of a average monster, want a mini boss the add some ac ans resistances with a hp boost, and items if they are humanoid, and you good to go. So maybe apply some of these to some enemies to get more out of them. 

I hope this helps. 

Here's a thought... have a group of 5 smaller enemies stats handy, and come up with a logical reason why they might join in the fight. If the players are steamrolling through conflicts too quickly have these extra enemies start pouring in to give them more of a challenge. But if you've found a good balance you might not need them.

My main campaign has 8 players in it and I know your pain here is a few things I've found help with making encounters a bit trickier for the party.

If you use the flanking rule then here is a little change I use for my game that I kinda stole from warhammer. I don't give flanking bonus to the party when they are fighting creature that are huge or larger arguing that in my opinion the creature is just to big to care about the little guy behind him. Only really helps with encounters that really on big beasties but I've found it very helpful to stop them getting advantage on every thing they do.

Environment is key if you can't challenge them with a really strong creature, make the weaker creatures harder to deal with in creative environmental ways. Traps, difficult terrain maybe even tight spaces can really make something trickier I've found.

Like someone mentioned already always keep some units in reserve it's much easier to add more mobs in if the fight is to easy then it is to take them away with out it feeling like your helping them.

As for turns taking too long all I can suggest is rule with an iron fist a while back I started doing my one call rule where is I announced it was a players turn and he or she wasn't paying attention after a few seconds with out a response I'd move on to the next person and they would lose their go, they very quickly learned this and I've had very little trouble since. I also try to push my players as much as possible to know what they are going to do before they turn as to not hold up the combat. 

I'd suggest being careful with just doubling hit points as It can end up just being a punching bag which sometimes isn't really a lot of fun for either you or the player's that said adding hit points here and there isn't a bad idea within reason.

Don't let them rest too often this is something I made a mistake with I'd let them rest far too much so every encounter they would burn all they're spell slots and pop all they abilities then just go "We want a long rest". The way I dealt with this was to simple take away the ability for them to long rest outside of a safe are like a town or encampment, I'd allow them short rests while out in the wild or dungeons but never a long one unless a situation where it would be safe presented itself. This helps a tone because I can throw a few easier encounters at them to wear them down and then by the time they hit big bads they struggle a bit more as they are running low on resources.

Hope this helps not the best advise but just some stuff that has helped me deal with bigger groups.

It's difficult, and I expect that in particular the problem you have is that enemies are just focus fired down too quickly as damage output tends to be very high in the first 1-2 rounds of combat. But I'd do the following:

Combats with 3 larger, more powerful creatures and an additional swarm of about 4-6 lesser minions is probably going to work best. Alternatively, one very powerful enemy (CR 9 maybe) plus 3 tough minions may work well.

Use terrain to your advantage, and ensure that the enemy have spells like Invisibility and Fireball to punish players for clumping up. You need your fights to happen over a larger area than you would for 4 players, so that they have to divide and tackle different enemies like smaller teams. You need all of the players to feel threatened, which won't happen if you only have a single large enemy, or if they can use AoE spells to annihilate clumped up groups of monsters.

Explain very clearly to the players that combat will work as follows:

1.  Players roll initiative and you will write the order on a white board, then display it for them so everyone can see.
2.  No talking to the player whose turn it is, and no talking outside of that (aside from a little banter if it doesn't slow things down)
3. When it's a player's turn, they must immediately move or declare their action.
4. The player has 10 seconds to either ask a question (for clarification) or declare their first action. A question might be something like "Can I see that enemy through that window."
5. The player should then make any relevant rolls and finish the turn.
6. If a player does not know what they are doing and the 10 seconds elapse, then they miss their turn.

I think I'd have to implement that system or something similar, as even if a player takes 2 minutes to take a turn, you're looking at 20+ minutes for a single round of combat.

If you have the ability to "wear down" the party before they get to the boss this can be a huge help. That is, they used all their spell slots in the first set of encounters and now against the big boss their "only per long rest" abilities are out of action.

You need to put them into a situation, either by time pressure or by the environment, that prevents long and possibly short rests, for this to work. A dungeon with routine monster patrols... areas without doors that can be wedged shut but are wide open with multiple entrances and thus unsafe... attacks happening at sporadic intervals... can all prevent the players from wanting to take rests. And this means that their limited-use abilities will be worn down little by little, even when fighting relatively easy battles. By the time they get to the boss, they'll only have a couple of spells/abilities left each, and will have to think hard about how to use them.

Large groups can make challenges easier to deal with for the group. However, you can scale up encounters and that will help.

One big issue though is, how many encounters will the group have in a day? If a large group uses all their long rest resources on one battle then they either need a VERY tough challenge or they will walk through it. However, at the end of the battle, they may have little or no resources left and decide to head back to the inn for a long rest and continue the next day. This is a problem because a party using all their long rest resources in one or even two encounters is far more powerful than a party that has to split these resources over 4 to 6 (or even 8) encounters.

Adventurer's League has to deal with this issue all the time since groups for a particular module can be anywhere from 3 to 7 players. In this case, most Adventurers League modules provide suggestions for scaling up and down the encounters to deal with everything from very strong to very weak parties. In addition, if magic items in the party or some other characteristic of the party makes them even more powerful then the DM can scale the encounters even further either by adding more monsters of the same type or adding something else that is thematically appropriate.

In your case, it sounds like you may need to scale encounters even further by adding more opponents. The problem with a few challenging targets is that they still only get the one action/monster unless you give them Legendary or Lair actions. On the other hand. a horde of low level creatures can even defeat a level 20.  The key is experience and practice so you are able, as DM, to add sufficient monsters to an encounter so that it is fun and challenging but not instantly deadly or unfair (unless that is what you are going for ...).

I have had great success utilizing the encounter simulator here on Beyond to adjust encounters accordingly.  Though, it's a rough estimate of what to expect, it will definitely give some ideas on how to properly up the difficulty.  Granted, I've never ran a large group.  However, one campaign I'm currently playing in has eight players.  Our DM hardly lets us rest and throws environmental factors in on the regular so by the time we make the encounter, we are already fairly spent.  Our last encounter was a single ghost and only 6 of us went to this particular area.  To save itself, it kept possessing each of us and the rest of the party would have to knock that player unconscious while the ghost used their skills to fight.  Eventually, we beat it, but we had three near death experiences if it wasn't for the paladin doing lay on hands at the last possible second.  It was intense!  We are level 8, by the way, and only one person had above 10 health.  Everyone else was directly at 1 hit point.  We went from that to yet ANOTHER encounter.  Thankfully, by that time the other two arrived and were able to help out and nobody died.  But, I'd say that is a good example on how to knock down your large party that thinks it's invincible.

This went across two sessions, so at the second session, everyone was showing up with backup character sheets in case their primary character died.  lol.  

First of all, the way numbers are balanced assumes that both sides have some access to area effect attacks; monsters that mostly have single target attacks, particularly if melee, will be less effective than expected, monsters that have good area attacks will be more effective, particularly if they can attack first. For example, a Mage (CR 6, which would be considered Easy against 8 PCs) who notices the PCs before being noticed can cast Greater Invisibility and drop a level 5 Fireball on the party, which will probably drop multiple characters, and a second level 3 fireball should be a TPK.

Other than that general note, doubling the npc groups (as long as they aren't all bunched up for convenient area effects) should work, but remember that the game assumes you can make it through 6 Medium encounters in a day, and if you don't do that many fights, fights will be very easy and spellcasters disproportionately powerful.

I deal with a party of 9, a common thing I find that works, say they are level 9/10, and I throw an adult black dragon at them they’re able to take it and tank out some of the hits it throws at them and deal back. The issue I come across there is THEIR damage is too high so what I do is adjust health of the fight accordingly to make it, an actual battle

At 9 characters, per RAW encounter guidelines, an adult black dragon is an Easy encounter for 9 characters; you'll need an Ancient dragon to be classed as a medium challenge.

Focus on abilities that (this is going to sound kind of sucky) take away a couple of player's turns per round. Upcast Hold Person, find a way to stun a couple of them, use Legendary Actions/Lair Actions to make it so the boss(es) of the encounter can have "multiple turns" (kinda)... Let those effects target different characters and last a couple of rounds each so that you're realistically dealing with a rotation of 4 of the 8 characters per round. Speeds combats up and makes balance easier. :)