It's probably the servers -- the game got released early because Gamestop didn't feel like waiting for the release date, so things are a little messed up.

Just go practice against Nightmare computers or beat singleplayer, tomorrow the normal servers will be up and I'll be more than happy to destroy your soul.

I'm getting the same. When I check the logs it says the games I'm trying to join are full, although the UI says otherwise. Who knows, I'll try again tomorrow night I guess. I also don't understand why they'd put a more descriptive error message in the log and simply display "failure to join game" in the UI... unless the log message is inaccurate.