I run a game with 6 people who are all interested in D&D, but all of us have competing demands on our time (professional, family and social obligations) which make it impossible for everyone to meet at the same time. We've been meeting for about a year and have completed a few one- and two-session modules as well as Waterdeep: Dragon Heist.

So what I've done instead is implement some complementary procedures.

Cadence

We don't meet more often than every 2 weeks. This is both so that I, the DM, am not in a constant cycle of preparation, and so that the players don't feel like they're always under obligation to do D&D.

Quorum

If at least 3 players and the DM can meet, we meet. If not, we find the next week that works. I use a scheduling tool so it's very easy for folks to fill out the next 6ish weeks of availability and then match up the schedules.

"Poof!"

If a player can't make it, we just suspend our disbelief about their absence. There's no narrative explanation for their character's absence; nothing adverse will happen to them. If they were carrying an essential MacGuffin, the MacGuffin is instead carried by another character. When the player returns to the game, they rejoin the party as if they had never been apart from them.

Because there's no narrative explanation for how this makes sense (it doesn't!), it requires everyone to let go of the details a little bit, and trust the DM to use spackle to cover over any problems.

In exchange for your disbelief, you're buying everyone some breathing room. If someone has to go to an out-of-town wedding one weekend, that doesn't stop the rest of the group from fighting the BBEG.

This dovetails nicely with longer absences, sudden absences (illness, family emergency, car trouble), or players which have tight constraints on availability, since you don't need to explain anything about why anyone's gone.

Depending on the tastes of your group, you might need to finesse this a bit. For example, we use milestone advancement and everyone advances at the same pace, whether or not they can make it. Sometimes this means that a character goes from level 2 to level 4 without playing a game. No one in our group cares, but other groups might have different feelings about how advancement works. The best way to figure out a solution is to talk to your players and make them feel like they're a part of the process of running the game.

Make self-contained adventures and change the number of parties

In my current game, I have some players who told me upfront that they cannot commit to a long campaign in case important real life business comes up. In total I have nine players.

What I did, was have a session zero with everyone (where already two players couldn't come, but they handed in their votes on important topics before). I said, everyone is part of a certain adventures guild and they can come up with their own reason why. Then I split the nine people up into four and five. And I do self-contained adventures of one or two sessions (I put the two sessions close together in the latter case, so that players can better tell if they can commit for both).

Now, if many players commit, I run the same adventure for two separate groups. If few people commit I run only one adventure with all those people. This way I can always get groups of three to six players.

The adventures guild gives a good reason for the characters to work together in changing composition and can also serve to explain why there is this changing composition. People can have signed up / been assigned elsewhere. There are a lot more jobs than just the one.

Of course, this means that one has to run extra sessions, but there is not much more preparation since you use the same adventure twice.

It is difficult, however, when your adventures are not reasonably self-contained. It works best if one quest is one session so that no one even has to commit for two sessions.

I actually run a game similar to yours. Mine is through a local library, but it also has a whole bunch of players (9, at most), and usually I only get a handful (6, maybe 7).

I have a few suggestions, which can be implemented in any combination. This might be a little focused on avoiding the root problem rather than the question you’re asking, but I’ve seen all of these work well to solve similar problems.

Talk to the players

This is a good idea anytime there’s a behavior issue. You can always say “hey guys, I think when we run as a whole group, it’s hard for people to share the focus” and ask them for suggestions. I’ve done this before, and my players generally say they feel better knowing their GM is a human, with human problems. If they have a good suggestion, I would try it for a session.

Absent players

My groups usually play one of two ways for this issue. In three groups, an absent player means the character stays behind the last place we rested. The absent players still get XP, but the rest don’t have to worry about dealing with more than one character.

The other group I’m in, as a player, is big (10 players). We treat absent players or new players a bit differently: time travel shenanigans are a thing so whoever shows up, their characters are there too. If you can’t make it, your character just isn’t existing right now or never did in the first place.

Either way can work, depending on the group. Again, talk to the players.

Side note: I prep as if I’ll get about 2/3 of the players that usually show up. The GM in the group I mentioned plans for about 7 players showing up on average, then changes that if necessary. Plan for fewer players then the average, but have adjustments for more players showing up (harder problems, stronger monsters) just in case.

Split the group

Originally by @linksassin in chat: link, scroll up a bit and then down for the rest of the discussion.

If you still can’t run a big group, you might try splitting the group. This doesn’t mean split the party, but it might (for me, when I’ve had to do this unexpectedly) mean running two or three sessions in the same world that have different subsets of players. Again, talk to your group. Please. Everything is easier if you talk to them, and listen to their responses.

Get a co-GM

Another person to help, whether they just help adjudicate actions for other players or they run one group through a scenario, can be a lifesaver here. In my big group, I occasionally get the GM for another campaign with a similar group (he plays in my campaign, I play in his) to help me look up a ruling while I deal with someone else. It saves time and keeps focus on playing.

Use a chat

You say you’re playing online. For my game, now that we’ve moved to video calls, I use the chat feature to adjucate multiple actions. The first person to say something over voice chat gets the focus, and I deal with that person before I move to another person. Anyone else who isn’t in on the current interaction who wants to add something or do something puts it in the chat and then I go through the chat in order and work with those players to resolve actions. We also usually put dice rolls in the chat so as soon as I tell someone to roll I can move on.

This doesn’t work quite as well in combat, but we use something similar for that.

Remember you’ll do fine

You will be fine, whatever happens. Players probably will accept “I need to figure some stuff out about our game, let’s try this and see what happens” or “here’s three things this player told me they had trouble with, how would y’all fix them?” I’ve done this, other people have done this, there’s plenty of ways to make it work.

Whatever happens, work with your players, be responsive, and everything will end up better than before.

I have had the same problem before, there are some fairly simple solutions that worked for me:

Have the players explain their own character's absence

Have players come up with reasons for why their character can't make it or why they have been absent. If someone asks you, say "I don't know, ask them". We had a monk who often wouldn't show up, and the reason they used was they had to go back to their temple for various rituals or training. Sometimes our wood elf ranger didn't show up, they liked to say they were tending to the forest and checking on the animals. All characters have other things they would be doing, it's not my job to know what those things are!

Start and end sessions in town

If every session starts and ends in a common location, then there is always an excuse for characters to skip the session or join up, they were in the town doing other things. If you have multiple sessions out of town, then you can say characters are coming from/going back to the town. This works great for doing multiple delves into a dungeon too.

Have all PCs belonging to a single organization

For example, if they are all members of the Iron Crow Mercenary Band then you can always say that the party that session was assigned by higher-ups, and if someone leaves or joins the party next session, well it was the officers that ordered the change, it's just like that. This gives you an explanation all the time, so you don't have to worry about "why would the elf go play with the deer in the forest when the necromancer's army is marching on the city??" and being mad at the PC. Instead, everyone can be pissed at the top brass's decisions, that's life.

Limit the number of players per session

Have a cap on players per session, and have a first-come-first-served system. If you can't run more than 6 people, then don't do it. Just let the first people who sign up play, or those who haven't played as much. Just be sure to give everyone a turn, instead of having the same 6 people sign up for every game forever in advance. Also make sure that if someone drops out, give others a chance to join. I think you have to do this for practical reasons, running 10 person games is ok some times, but it takes a lot of prep and it's very different. Not really a chill weekly game for me.

You've written about "[finding] a narrative reason someone else can come along". I have practical experience with this, and I can tell you that what works the best is not doing that.

Instead, I just rewrite reality.

Let's suppose that Alice's player is out of town this week. What I say is: "Alice has never existed. There have been three of you this whole time. You fought those ogres last session with just the three of you. It was just the three of you who found the underwater cave and talked to the water dragon. Any memories you have of Alice are just your imagination."

The next session, when Alice's player comes back, I un-rewrite history and narrate that Alice has been there the whole time, including the most recent session Alice's player wasn't there.

If Bob's player is joining the group for the first time, I do the same thing, but in reverse. I tell everyone that Bob has been with the group the whole time.

I like this approach because it sidesteps a lot of problems. I don't have to think of narrative reasons why people's characters show up or leave at surprising times. I don't have to spend game time on explaining those reasons. If a new player shows up, I can just write them into the story and not have to deal with the awkward "hello, new person! we've decided to trust you unconditionally for no particularly good reason" scene. I also never have to deal with one player playing two characters.

Sometimes players are surprised by this at first, but once I explain it, it works fine.

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In most games, it would be considered pretty rude if someone showed up to play and you told them they couldn't play because their character was staying in town.

But it sounds like you're already doing that.

With that in mind, it seems like there's an easy solution to your problem: you just tell your group that the first five people to show up to the game are the ones that came along to the adventuring party.