Clanning is pretty dead. In fact, it was never really alive for Sins, not in the sense of having numerous clans that competed against each other in leagues and on ladders. The only clan where I see more than one or two people on at a time is _|~ Defiant Unity. Members of [DT] Dream Team and [_]-Brothers in Arms are still around, too. I don't think any of them are really recruiting and there isn't much of a reason or purpose to want to join a clan now.
If you are interested, then, as always, the best way to join a clan is to make a name for yourself and to get to know members of the clan. Make friends with them first and then ask. Come play in the 4v4s and 5v5s on Diplomacy to meet them. That really is the best way to join a clan in any game--get to know the people first. Hang out in their IRC chat channels (or wherever they hang out), etc.