I'm not sure if it's just me, but every time I go into the game lobby it shows all the games in gray. I ran into a way to get around this and join games.

Hit the view gamercard button, back out of the profile, then view it again, you should be able to join in progress. I'm thinking this is a bug since every game I join it seems that the people I'm with are doing the same missions I'm doing. Anyone run into anything similar?

The only time it has worked for me is when I got disconnected from Live and then reconnected and it allowed me to join nearby players. That's it though :( Hope it's fixed soon.