The way we do it is to have a core group of four players (including the GM) who we know are committed to the game. We schedule around the core. Once we know we’re going to game, we let everyone else know when the game will be, and that we’d love to have them here.
In our current game, this means we have four to seven players.
Sometimes one member of the core won’t be able to show up but we know that everyone else can; we will occasionally go ahead. But if two members of the core can’t show, there is no game.
If we’re missing one non-core player, their PC becomes a group puppet; if we’re missing a core player, their character goes off elsewhere until the next game session. If we’re missing multiple non-core players, obviously their characters are off exploring something else, because they aren’t here now. I’m sure they’ll catch up wishwith us sooner or later.