If I am looking for something useful, then it has to cover my interests but then again the forum should be active, otherwise what is the point of talking to admin alone? Other than that, I join forums if there is an exchange need or if I want to advertise my forum.
I join forums when I see the need to do so. It could be because my friends are also part of that community or as a result of liking the topics that are being discussed in that forum. Also, working on the forum project could also be a reason to join any forum, but the reality is that I only join forums where I feel that I have something to offer.
