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What exactly are you trying to do?
If you want categories, you can create those in the Admin/ Users area. If you want to place certain users into specific categories you can do that in Admin/ Groups. If you want people to self-assign to your groups during registration, you probably need to hack the register.php area
Beyond that I'm not sure what you are looking for...
Pax vobiscum,
...mark
may the road rise to meet your feet!
http://treemagic.org