Profiles and Matchmaking
Your profile page, and other users profile pages, are your central hub for matchmaking. Viewing anyone's profile will show you a list of co-op games they currently own, and whether or not that matches up with your existing list. If it does it's easy to send them a message to play just by clicking the "PLAY" button.
You can find user profile pages linked in various places across the site like the comment sections of articles, our forums, and on blog entries. But if you aren't sure who to play with, or who's a good fit you could always have the system find other community members who it thinks would be a good match for you.
In the right column of your profile page click the "FIND MATCHES" button under the Co-Op Gamers Like Me option. From there you can see other members' games, and send messages for play.
Alternatively if you are just looking for someone to play a game, say Halo 3: ODST for instance, all you need to do is click "MEET OTHERS" underneath your games list next to Halo 3: ODST on your profile page.