You should also note that a number of the Sins clans will only let people join once they are of a certain skill level and are known to that clan from being involved in games with/against them. The tops ones (DT and [_] in my opinion) are very selective. Others are much more open, but some have formal strucutre rules etc...

You could just play the game, read the forums and ask people questions in the lobby.... It has worked for me. I am not currently in a clan, even though I am friends with most of DT and [_] and other good players...