I've found myself hosting a large monthly game made up of mostly new players. Out of a growing mailing list of 40+, we regularly have 10-12 players in a game. It's chaotic, it's messy, it's maddening to run... and it's a blast. 

My group is made up of busy, professional adults who have never played an RPG before in their lives. Or they think they have, in a Zelda or Elder Scrolls game, but had no idea the depth and richness of content and experience offered by tabletop RPGs.

The social proof is almost alarming. They'll sheepishly admit to other friends that they've "been playing D&D" only to be met with intense interest. It's like Fight Club.

And as busy adults, they have obligations: kids to feed, dates to go on, work to finish up etc. So our game is drop-in, drop-out friendly. It's not uncommon to have a few first-timers, so I've boiled down the "what you need to know" rules into a couple of Keynote slides I can run through in less than 5 minutes.

Obviously there are problems with having a lot of new players. Do I have to explain - again - for the third time in a row - which die to roll to hit and which one to roll damage? No you can't use Insight to find the treasure. No I have no idea which miniature is yours; pick one. Stop eating those.

But it's still a ton of fun, and with D&D experiencing a bit of a comeback in popular culture, it's worth reaching out to people who don't normally play with you. Here are some tips that help me out in games this large:

Planning The Game

• Use Communication Tools: a large group that doesn't see each other often needs good asynchronous tools for communicating. Email threads are good; Slack is better. Find a way to keep everyone up to date. I'm considering tools like Meetup.
• Defray Costs: I've asked players to chip in a small amount on the D&D Beyond Legendary Bundle. A game shop owner told me other DMs offer Inspiration in exchange for Monster Menagerie booster packs. Whatever makes it work - a large player pool is a good opportunity to be more lavish in your preparations. 
• Be Inclusive: If you grew up in the 70's and 80's as I did, you'd be shocked who wants to play D&D today. People from multiple age groups, genders and backgrounds want to play in my games, and they will in yours too. Look past the traditional "RPG nerd" stereotypes, and don't be surprised when generations reared on Harry Potter, LOTR and Marvel on the big screen want to try their hand at classic RPGs.

Campaign Building

• Rely on tropes: This is where the D&D system really shines. The game's status as Granddaddy Of RPGs will ensure that every single player, whether they're brand new or a hardened veteran, will have some kind of familiarity with Orcs and Dragons and Goblins. Don't avoid putting then in a dank dungeon or in a Kobold ambush for fear that it's too basic or unimaginative. Well-known fantasy tropes help you cut down on exposition and keep the game moving.
• Use an Episodic Story Format: It's easiest to work with a shifting group of players if each session is self-contained, but related to a larger story arc. This way if players miss an episode or two, it's easy to explain what happened and get everyone back on track. They do this on TV for a reason.
• Railroad Like Crazy: Keep huge groups on rails. It's not worth a 30 minute diversion in pursuit of a red herring if most of the party is staring at their phones. Keep the story points relevant and obvious, and if you're going to use amusing interludes or side-stories, limit how much players can interact with them. 
• Limit Unnecessary Choices: In The Paradox of Choice, Barry Schwartz argues that while we "have more choice than any group of people ever has before, we don't seem to be benefiting from it" The same is true in D&D: offering too many choices creates an obligation on players to try them all. Don't; instead, coalesce related options down to a small set of choices that are truly distinct.
• Always Have Premade Characters: New players will stare, bewildered, at the Player's Handbook and will, after hours, inevitably roll a LN Tiefling Sorcerer with Draconic Bloodline. Don't let that happen. Instead, have a few classic, helpful and easy-to-learn characters rolled up and ready to hand to new players.

Running The Game

• Optimize for fun: As a Dungeon Master, your job is to ensure that everyone is having a good time. That's it. You don't have to make sure the rules are followed, or that the players discover every nook of your precious dungeon, or that tons of phat lewtz are efficiently plundered by minmaxers. Keep it fresh, keep the game moving along, don't get lost in the weeds.
• Use Visual Aids: I usually use Keynote/Powerpoint slides to illustrate basic scenery, rules, key monsters and treasure. Visuals are a powerful way to focus a large group's attention on a single topic.
• Overdo It: If you usually take a light touch with your roleplay with small groups, you'll need to ham it up. Larger groups need some action to keep them focused - don't be afraid to lead the way with third-rate theater.
• Scale Up: It takes more enemies than you think. Use the Dungeon Master's Guide advice to scaling up CR for larger groups, or just throw more monsters at them than you think you need. Larger parties are going to scale incredibly well once newer players figure out how powerful their abilities are. Let the Bugbears fly.
• Initiative buckets: Group initiative rolls in buckets of 5 (20 to 25, 15 to 20, 10 to 15, etc). It makes any manual system saner. 
• On Deck: Call out the player who is "on deck" to figure out what they want to do while resolving the current player's turn. 
• Roll both dice: Train players to roll attack and damage dice at the same time. Train them to deal with the "which dice should I roll?" instinct in the on-deck phase.
• Stay Limber: Players will come up with crazy stuff. Let them - that's part of the fun. As a DM, you need to stay flexible and allow their ideas to impact your plans. 
• Set Boundaries: Larger groups will test your limits. Establish firm limits and show them the consequences of their actions. 
• Don't Sweat The Small Stuff: If you need to peek at the Player's Handbook every five minutes, a large group's going to eat you alive. Use your judgement, be authoritative, and learn to brush off small details in the name of expedience and fun.

That's all I got. DM's, how do you deal with your larger groups? Any tips for onboarding noobs? How do you deal with gigantic battles?

+1 to all this. Sounds very similar to my experiences. I’m running two games that started off as shy newbs that are now fully out and proud D&Ders. My works group even have it on the work events calendar so the whole (1000+ employees) company can come and join. 

I‘ll add to this line ‘don't be afraid to lead the way with third-rate theate’. I was petrified of RPing, but turns out everyone is, but that’s what they’re here for. Doesn’t have to be Oscar winning, just ham it up. Accents, arm waving, sound effects etc. Do it all with a cheeky grin and a glint in your eye and you’ll have them rolling with laughter or hanging onto every word.

Above all, don’t take yourself or the game too seriously. I’ve had a couple of times where I’ve been frustrated with the group going off piste and had to hard reign them in and both times killed the fun and the atmosphere slides  

Incredible stuff, thanks for the post. 

Wow. Well written and argued post. Should be a sticky post. Most if not all applies to small groups as well. Thank you. 

Wow!  Such great advice!  I'm now a year into a campaign that has grown from 6 kids to 14 kids (ages 9-14)!   Now I have a very real need to pull out all the stops on large group ideas.  We play at the local library one day/week for 2 hrs.  So, as you might imagine it can be either a really great session or utter chaos.  Here are some things I do that may be helpful to folks.

1. Encounter sheets with pre-rolled initiative for the monsters

2. Spellcasters MUST look up their spells and understand them between turns and be ready.  

3. I'm very much starting to railroad.  But in between large adventures we have "downtime in town" where they can cut loose and do personal projects.  I try not to roleplay these in too much depth, but instead give them the results of their actions while split up.  Then I get them back to a central area to get things rolling again.

4. Have the kids run groups of minion monsters while I run the big bads.  This keeps them engaged while they wait for their character turn. 

5.  Third rate theater ALL THE WAY!!

Thanks to everyone for sharing ideas.  Can't wait to incorporate them next week!