Quoted from the site in my signature ...
7.2 UT Clans
What is a clan?
Clans are the "clubs" of computer gaming. A group of people, who decided to join and play the game together. They will identify their membership by carrying a "clansign" in their nickname.
A clan will usually have regular training sessions to improve their teamplay and play matches against other clan. Many clans play in organized leagues.
Should I join a clan?
A matter of taste. Many clans are a nice bunch of people and it's real fun to play with them as well as against them. Others suck with their arrogant attitude and behaviour.
Do you play a lot and enjoy playing in an organized team? Enjoy talking about the game and sharing your tricks as well as learn tricks from others? Then try it and decide for yourself.
How can I find and join a clan?
Finding a clan is easy. Look for players with clansigns on the game servers, that you usually play on. If you get along well with them, ask them if they are recruiting and ask for their e-mail, ICQ or clan website. If you are good, maybe they even ask you.
A large list of UT clans can be found on Unreality:
http://www.unreality.org/clanlist
For other clanlists see below.
Most clans have a website, where you can find information about the clan and how to join.
Some clans will ask you for a try-out before they allow you to join, to check, if you "fit" into the clan with your skill and attitude. I'd recommend choosing a clan, that has about your skill level or slightly above. You won’t have much fun, if the others are miles ahead or behind you.
Clan Lists
Unreality Clan List:
http://www.unreality.org/clanlist
UT World Clans:
http://clans.utworld.net
Clanlists for various games 1:
http://www.clanlist.com
Clanlists for various games 2:
http://www.clanbase.com