Hey all,  (I am new to Dm'ing)
My players are wanting whenever they get surprise on an enemy (Which they do a lot having a rogue which is super stealthy) Wanting to setup attacks on players, Having a couple of players attack at the same time (We have 2 PC's with Bows). I am wondering if there is actually a mechanic for this, If dm's let players do this often if not how maybe to deter them from this? 

Should I just talk to them and just completely Scrap them doing this and use the standard surprise? 

The only standard way to delay an attack once initiative has been rolled is to take the Ready action and set a condition based on someone else attacking, e.g- "I draw my bow but hold my attack until <insert other character name here> attacks". But keep in mind that the standard rules only currently allow you to make a single attack when you do this (which personally I hate and overrule, but if you want to stick to the rules as written it's one attack only).

That said, it is up to the DM when (or if) to put the game into turn order and roll for initiative, so if you feel that the party have setup a really good ambush, and the enemies haven't spotted the trap, you could just choose to allow each player to take one turn each in any order, then roll for initiative skipping the surprise rule (since you choose whether to roll for surprise, and the enemies should know they're under attack at this point). If you go this route take care of any character that has rules that rely on surprise or the first round, as by granting a free "pre combat" turn you've essentially bypassed these; you could allow them to trigger in that free turn but again this wouldn't be rules as written.

This makes a lot of sense! Thank you very much :) 

Just a couple comments ... make sure you are running surprise correctly (unless you want to run it differently on purpose) since if an opponent notices any attacker then they are not surprised even if they didn't happen to notice the rogue. I usually find that a super stealthy rogue doesn't help much in terms of the whole party getting surprise since if the opponents notice any of the less stealthy characters they aren't surprised.

In terms of coordinated actions that the party tries to set up out of combat, I use a house rule.

I have the party decide which character is going to do something to start the encounter - open a door, make an attack etc. This character will then go first. You can do this either by assigning this character the highest initiative since their action starts everything happening or by moving down the initiative order to where their roll occurred and proceed from there. The first round is over when their turn comes up again.

The reason I use that house rule is the following:

eg 1: if you have surprised some enemies, you set up an ambush, they didn't notice and Joe is making the first attack then it doesn't make sense when everyone rolls initiative and Joe is last. Everyone else in the party can take their action and even the opponents will no longer be surprised (they will have had their first turn) by the time Joe gets a chance to do something ... and by that point the action they take could well be something different since whatever they were going to do to start the encounter may no longer make sense. I find that for both the players and I the logic falls apart so I find it better to start the initiative order with whatever action starts the encounter - whether that is opening a door, kicking over a table, making an attack - no one has any reason to do anything else until that action happens. 

eg 2: Involved in an intense negotiation, one player character decides to take an aggressive action that could result in combat, NO ONE in the encounter has any reason at all to do anything except keep negotiating until that character takes their action to trigger the encounter. However, as written, everyone else could start making attacks or taking other actions before the trigger that caused initiative to be rolled even comes up.

So for encounters like this where there is a very specific triggering action taken by a player or an NPC that requires turns to be resolved in order - I use a house rule to start with that player.

In your ambush example, whoever was designated to attack first, makes the first attack then everything else is in initiative order.

Wait, were you letting the players make/ready attacks before rolling initiative? Because that is not how it works RAW (though the rules don't exactly cover every case).

RAW surprise is actually hard to get, I'm surprised there isn't a single low stealth character in the party. A single bad stealth roll prevents surprise entirely.

Quote from DxJxC >>

RAW surprise is actually hard to get, I'm surprised there isn't a single low stealth character in the party. A single bad stealth roll prevents surprise entirely.

It's not that hard if the party is willing to work together to improve their chances; pass without trace is a fantastic boost and fairly easy to include in your party's mix of abilities. You can also ask other players to take medium armours that don't disadvantage stealth in armours that do, either that or have less stealth party members hang back so they're not part of the group rolling for stealth, or the DM can have them roll but grant advantage for being distant.

There are definitely options to make it work smoothly, the main problem with the 5e surprise rules is that they're scattered all over the place and not terribly well explained.