I join groups. I'm in about 5 (I can't be bothered to check), but am only really active in the <url=http://www.escapistmagazine.com/groups/view/Stargate-Fan-Club>Stargate Fan Club (of which I'm sort of 2nd in command).
I think they're a great Idea. They're perfect for organising multiplayer matches and tournaments. If you put up a thread on the forums showing the unboxing of your new Deth Glider model no one would see it, and most people wouldn't know what the hell you where on about, but if you posted the same thing in the Stargate Group then you'd get a lot more discussion and jealousy (the true point of unboxing photos).
They're also excellent for organising competitions. I recently ran a contest to win a game through the Stargate group. Andy Chalk also (very kindly) posted a notice about it in the PC gamers group. That way a lot of people got to know as it was promoted to those who would be most interested.
They free up the forums, forge a closer community, and bring in extra traffic. I'm glad they where implemented.