Keep the Paper Flowing
Keeping track of a bunch of players is a logistical/bureaucratic process. You need to know who your player pool is, how to contact them, and a way of tracking who has played when. I recommend having people sign up by answering a survey made with Google Forms. Experience using surveys with students suggest that you need to limit yourself to 10 questions or less and any question that requires more than a one or two word answer counts double. Here's what I might ask:
- What is your name?
- What's an email address you can be reached at?
- What is your Google+ handle?
- If you have a FLAILSNAILS PC, what is theire name/race/class/level? (You aren't committing to playing that exact PC and no other, I'm just curious)
- Tell me one weird thing about your PC [this would count as 2 questions]
- Any special concerns about the game? I.e. schedule wonkiness, are you hearing impaired, is there a kind of monster that you really can't deal with? [also 2 questions]
Google Forms allows you to dump all that info into a spreadsheet, to which I would add columns to track who played in what game. That way I can tell at a glance that Bob has played 5 of the last 8 sessions, so maybe someone else needs a chance, meanwhile Christine has been on the list since the beginning and still hasn't got to play. Speaking of which...
There's More Than One Way to Make a Party
I used several methods to decide who got to be in any particular session. Random selection was a common one. Keeping an eye out so that more people got a chance to play was another. However, I also had really good luck with some hybrid methods, such as keeping one player from the previous session and randomly select three others. This allowed for some continuity of play. A couple of times I picked one player (randomly or not) and let them recruit the rest of the team. This worked best on the occasions that I got emails from players who clearly had an interesting agenda for the game.
You need a clearly labeled channel for official communiques from you to the entire player pool. Obviously this blog was handy for that. You also need a central venue for players to talk about the game, like G+ or a facebook group or something. Also, the use of a single regular drinking establishment in the campaign combined with the carousing rules encouraging PC inebriation worked really well in allowing me to regularly broadcast details of the adventures that would otherwise be hush-hush. If the players spent hundreds of gold expressly to get blotto and earned XPs in the process, then there was no room to complain about me occasionally exposing the secret results of their session. This is important because you want enough info out there that the next party will have one or more ideas what to do with your dungeon.
Multiple Routes to Trouble
If I did one smart thing in setting up my dungeon, it was taking inspiration from the Caves of Chaos in terms of the number of ways into the adventure. Fresh groups knew they could try one of the entrances no one else had and find a fresh new bit of fun waiting for them, while veterans could move quickly through previously covered ground to reach deeper levels and more troubles & treasures. (By the way, if it hasn't been used I totally call dibs on Troubles & Treasures as a title for something.) And go ahead and make some of the entrances a bigger pain than others. Two of my favorite sessions started with players who decided to enter the most flood-prone sea cave and the time a group excavated the rubble pile to find a new stairs down. Sometimes to have an adventure you gotta do things the hard way. Heck, start with an obvious but magically sealed alternate entrance. It will drive players crazy.
Simple rules, simple setting
If you want the largest possible player pool you can't really make the larger milieu the star of the game, nor can you use a system where building a new PC feels like homework. Obviously, my setting mattered a lot to my game, but in a way that unfolded naturally through play rather than requiring significant briefing ahead of time. Also, try making a shared Google Doc with absolutely everything needed to make a new PC for your game. The shorter that document is, the better.
And don't run a system where you have to look up a lot of stuff all the time. Working through mechanical problems seems even more annoying when playing with people online. Better to run a dumb system you know down pat than a great system you're still struggling with. And remember, when in doubt give any vaguely plausible plan a 2 in 6 chance of success, but a roll of six means things go ridiculously bad for the party.
Maintain your dungeon
PCs in dungeons are like preschoolers in a library: lots of people have fun but when it's done a bunch of shit has been haphazardly rearranged and there's bodily fluids all over the place. In addition to noting monsters killed and treasures looted, it's extra important that you track any other changes: marks left, blood stains, traps disassembled, burnt out torches abandoned, etc. Oozes, vermin, and kobold janitors can clean some of that stuff up for you, but the players will eat it up if you leave traces of previous expeditions about the place. And once in a while shake up your dungeon status quo: move some monsters around, add a new trap in a previously-explored corridor, have an umber hulk or purple worm burrow some tunnels making strange new connections.
Well, that's all I got in me at the moment. Maybe some of my supercool players will chime in with what worked and didn't work for them.