I can also suggest taking turns. In our current group, we have about... 10 players, who are more or less active. When we're running, I (or the two new gamemasters we have) put up a message and the first 3-5 people who are interested participate in the run. It also makes interesting dynamics when players have participated on different runs and have different knowledge of different plotlines that are occurring around them.
The drawback is less karma/nuyen for people who run less, but you can give a "monthly bonus" even if people haven't been playing to balance things out a bit.
