You Join by clicking on Join Group. A group master (no not moderator, Master!) will validate you. From there on you are in, go into the forum and look for threads for custom games which will have a poll. To get notifications of posts, Start tracking the group. If there isn't a poll or thread and you'd like one, go ahead! Polls are there to check if people are interested in playing the mode or game with you. When you can find people to play with, team up with them! Share your steam in the forums when you create a thread or PM a person so you see who's online and who can play and so on!
Inside the Forum which you can find by looking at the Forum tab below, you can also report players who will then get 'stickied' on the Wall of Shame.
People who have nothing better to do in their pathetic life than to come here and post how bad BO2 is and how bad COD is may do that, it serves our amusement. Responding is forbidden for members of this group. Comments can get deleted if seen necessary. MODDB rules apply here as well as everywhere else on MODDB.
When everyone in the party agrees with something, do it.