I'm in one group, but we have three different games involving different people. This really came about because one of our players (let's call him gamer E) has developed an extremely full schedule, and even when he says he can make it sometimes cancels at the last minute.
Game 1: Gamer A is the DM, gamers B - E are players. We only schedule this one when gamer E can show up.
Game 2: Gamer B is the DM, gamers A and C - E are players. We try to work around gamer E's schedule, but we schedule this one once a month even if gamer E isn't available. We take turns playing his character when he isn't around.
Game 3: Gamer C is the DM, gamers A, B, and D are players. Gamer E has nothing to do with this one (I'm not sure he's even really aware of it), which means that it is much easier to schedule and allows the rest of us to get in more gaming than we'd be able to if we felt that every member of the group needed to be a part of every game.
On average we play each game once a month. I wish we could play more often, but it just isn't feasible.
Morrow