[quote=Jorick]
It's a thing that some RP groups think up because they're silly and it amuses them. There's no official rhyme or reason to the them, no standard way to join one. It's essentially a way for people to broadcast "hey look at all these friends I have" by listing their friends as their 'family' in their signature.If you want to join one, the simplest way to do so is probably to find one that exists and become friends with some/all of the members of the group. That or you could do it the other way around, make friends through roleplaying and having fun and create your own silly little forum family with them. Either way it just comes down to making friends, so go do that. 
[/quote]

It took until this post to figure out what the OP was talking about. Maybe I should turn sigs back on to see why people are so riled up, but my experience on RPG has been almost blissful since I turned them off.