I know that one of the other GMs I know does a lot of "invite only" RPs to ensure that there are people who don't disappear and such, but the problem with that is that you can't get a lot of new people to join up. Unless someone was to set up a sandbox RP and use that to find new people for invites, they would always be stuck with the same five or so people. Also, that would probably be quite a bit of work.