J1-J5(J6, 7....depending on where you are) is age. J1 is the oldest, and the higher the number the younger the racer.
SL is a single gate, the set up is a little different, and it's a "tighter" course, with more turning. GS is two gates with flags conecting them. they are more spaced out and have you making larger turns.
Yes anyone can join USSA, it's not free. but you don't have to quilify to join it, but you might have to be on a registered team but i'm not sure