My large group D&D has gotten to be quite large to the point where we can no longer take any new members without splitting the party into 2 groups. Currently, in meeting tomorrow there will be 12 PCs of varying experiences. As a result, what would probably take 2-3 hours for a smaller more experienced party will potentially take 4-6 hours, but I could still be wrong. But with such a large group, maintaining everyone's attention and making sure that everyone is on the ball is a rather difficult task. However, while it is difficult to maintain in-game, it is also quite difficult in planning encounters for the party as well. Currently the party consists of 12 2nd-Level characters which puts their party level at 5.2 (meaning they should be able to handle a CR 4 or 5 with little issues), but that level doesn't reflect their ability when many of the CR 5 solo monsters could one shot most of them. So what can be done?
While tomorrow is the first time that I'll be running a campaign with 12 players at once, planning was a little more difficult. I considered throwing some larger creatures at them, but where they're headed they probably wouldn't reasonably encounter something like that. At the same time though, I don't necessarily want every encounter to be a bloodbath just because so many monsters are needed to balance out the encounter. Altogether there really are a lot of things that need to be planned out and handled so that the encounter seems challenging and so the players stay engaged all while trying to keep everything interesting.
Challenging the Party
With such a large party it takes a lot of weaker enemies to make the encounter feel challenging without making it overpowering. In looking through Monster Manuals 1-3 I started looking for CR2 and lower enemies for an encounter or two that would create a challenge but not overpower and found that even trying to incorporate a few CR2s into the mix could create a bad situation for the party. Normally as a DM you don't want to have party members die right away, despite that it happens, but at this point I especially don't want to make it happen because the characters haven't had time to develop and figure out who they are yet (but that's another topic of discussion). While the party is a little weaker on the levels side and where the physical classes like Barbarians and Fighters have more of an edge because they start out stronger than the casters, it can be difficult to find a balance where the casters don't get destroyed and the physical classes don't wreck everything in a single shot. It really is a challenge trying to figure out the best way to address the best combination of enemies (be they strong or weak) to create a challenge, but regardless of what you decide there has to be a viable reason that they end up in that encounter (or set of encounters). An encounter without a plot point is really just a random encounter and if it is a large encounter, then really it just may feel out of place in the story. Yes, it may be another challenge to incorporate a large encounter into the story multiple times, but it's all a part of being a good DM.
Keeping the Players Engaged
What I've noticed with my large group is that they get very easily distracted during encounters especially if they have their computers as a resource. As a result, plot gets repeated, player turns take longer than they should, and overall everything takes longer which causes players to become disengaged. I've looked up tips to help this along and consistently the 2 most common solutions are using a timer and telling everyone who is on deck the next initiative. The timer essentially makes sure that players are on top of their game and makes it so they are ready on their turn. The timer starts when their turn starts (usually between 1 and 2 minutes) and they have that timeframe to begin telling the DM what they want to do on their turn. If they fail to complete that in the timeframe, the next person goes and that person that was not prepared can jump in whenever they are ready to act. This could be used to punish the player for not being prepared (especially if enemies are next) and this also creates a touch of realism as in a real battle you won't have a ton of time to strategize on what your next action will be.
The "on-deck" initiative is a nice starting point for parties that tend to be a little slower as it at least gives the next person in order a heads up to get ready with their actions so when their turn comes up they're ready to go. I would start here with a larger group just because so much can happen that you do want to give them time to analyze what has happened to select their next move, but if it does become a problem that things start to get slow, then the timer might be a good next step.
Finally, one of the largest issues my large group has is being distracted. One or two will be on their computers on Facebook or surfing the internet, some will be texting, and others will be having side-conversations, all of which distracts them from the encounter. As a result, when their turn comes around they aren't prepared and slow down the encounter creating more time where people distract themselves. In especially large parties, distraction is the biggest killer because when there are 11 players and 13 monsters that go before your next turn, it's easy to zone out or get distracted. A solution to this might be asking that players refrain from getting distracted as best they can during encounters and to try to "go off the grid" in a sense during encounters.
Keep Things Interesting
While this idea really is another form of keeping the players engaged, it really can be something that stands as an alternative to combat. A good storyline with varying levels of NPC interaction can force players to solve some problems without having a drop of blood fall. In my opinion, any campaign should be interesting enough that you could have the players never get into combat and they would believe that they are in a fantastic campaign. This doesn't mean that combat won't find them or that it isn't good, but the storyline should be as important as the combat that ensues from it. As I've said before in Findining The Right Balance if your party enjoys combat more, then add more combat elements, but still make those combats relevant to a plot just so it doesn't seem like an unending gladiatorial arena. On the same hand, if your party is more of a role-playing crowd then perhaps conflict and combat will find them more than they seek out combat, but both elements should be in the adventure to keep things interesting and engaging.
Avoid Cross-Talking
When my large group gets into a town, they almost instantly split up and get lost from each other, which is how they roll (they inevitably find each other in a few hours), but during these times, someone usually decides what actions they wish to take and a single person voices it. This sort of mentality works really well when the group is trying to make a decision and ultimately can be used so you get a final decision from one source. The party should select a person to be their delegate that tells the DM what the party is doing so there isn't a mash of "I thought we were doing this"s or "we forgot about this". Whatever the delegate tells you is simpler than collecting the opinions of every player at once. The other solution that I tend to do is I treat the decisions like a procedural hearing where I open up ideas for discussion and once a general consensus is made I ask if there are any objections or additions and then roll with the decision. The issue with either method is that they aren't really timely requests, but outside of combat time isn't always of the essence (in fact arguably, discussions are real-time).
Regardless of what many sites say in such that large groups should be avoided for all DM's and that anything more than 4 players is too many, it certainly doesn't make that true. I've found that while running a campaign with 2-4 players covers most of the bases for class needs, the dynamic created from a larger party creates some very interesting situations. Yes, it does require a little extra work and it can be quite time consuming, but ultimately your story should be adaptable to dealing with any reasonable amount of characters. While currently I'm most comfortable in dealing with 12 players maximum, this doesn't mean that down the road the party won't split up and find different situations or that a situation could arise where the party must create mini-parties in order to continue (arena battles, challenges, etc.) As long as you are comfortable with the party size you're running then that is what matters. There are plenty of tips for dealing with larger parties and this article only scratches the surface, so find what works and see if that increases your comfort level to allow for a larger party.