I don't think there's a way to fix it in Halo Reach anymore. But i'm very glad to see that in Halo 4 joining sessions in progress will be possible. That is as long as quitting won't be punished anymore. Because in my opinion, you can either have no join session in progress like in previous Halo games, but then punish quitters because it makes a big difference.. or you can have join session in progress and don't punish quitters, because then it won't be that much of a difference anymore.
Also, i hope that when you don't get punished for quitting and do quit out of a game, you won't be put back into the same game 100 times.