So Im playing on the XBox and whenever I try to join a co-op game the 'join' button is greyed out and the other players name is in red, for every single game listed in the browser. I looked through my settings and I dont think thats the problem. Am I missing something, am I just stupid, anyone else out there facing this problem? Help!

dont start a game first go to options and in online category make sure you have select co op, not lan. Select xbox live under the next option. Then start game. I hope that helps. 

I'm having the same issue, and my settings are definitely set to cooperative / xbox live.

The new patch that came out for PS3 anyway is making shit go bonkers. My buddy and I couldn't join a game because of it. As soon as either of us would try to connect to one another's games it would just disconnect. He said he'd just downloaded and installed a new patch for the game that I hadn't received yet. He deleted the patch and blammo, shit was working silky smooth again. 
 
Bad patch perhaps.