People played tonight, it was packed for the most part. I left earlyish though. If you are not sure you should check out CAGHalo friendlist and just look who is playing. They did make it private later...which I don't really see why. Think to stop some kid from playing, but whatever.
That's what I do when my invite disappears before I'm able to go through. If it is private just send a message to someone who is playing Halo 3 in that list to get an invite.