Even if the question is old it is not dead:
I am a member of a college club dedicated to RPG. All in all, we are maybe 40 to 45 people meeting every week (the same weekday since 1988!) for gaming RPGs, and many of us are GMs in one or another system while also playing under other GMs of the same group. Players also often play in more than one of the groups, so there is a similar unreliability built into the very idea of the gaming club:
Not every player can be there at all meetings (we are students and alumini, so some do have to work or study), and even if they are there, some of the players from last week might not show up at your table for they favor a different system this week.
On the other hand, it is not a given, that a GM will always offer one of "his" groups each week, as he might want to play one of his characters in a different GM's group or try out a new system once in a while.
To keep everybody up to date, some of the GMs keep Forum posts. I ask someone who was there the last time to recap for those that missed a few rounds. Others again just roll with giving a recap themselves ("You have been hired to do X in Y, last week we left here. You managed to achieve Z.")
Group XP have become a standard for most games, as it keeps PCs roughly on the same level, but it also can result in strange looks if one had been caught up in other groups and then return to the same table as before and realize one has to spend (insert arbitrary large amount of levels/XP/Karma/seasons) to improve the character.
Because of the unstable attendance to tables/systems, we tend to GM in session chunks, that try to keep a larger plotline in at worst 3 sessions, maybe with larger meta targets (arbitrary large like: Explore the Wild Lands!). As a matter of fact, this did lead me when choosing which campaign in Pathfinder I would try this semester: Kingmaker allows pretty easy possibilities to stash away PCs not present in the trading post/his kingdom role when the player is not present.
However, as the club has usually a large enough attendance to make several game groups, we have very seldom encountered a situation where we couldn't shoehorn together at least some game or another. With a much smaller peer group, this wouldn't be an option, but having some tons of one-shots or side campaigns/adventures might at least help for the days just a few show up.