the ones that are a series, run by someone like TSSCC (tri-state sports car council), WAI (wisconsin auotcrossers i...) or INR (indiana region scca) are all open to anyone. Usaully about 30 bucks to compete. the ones run by PCAM (porshce club of america) are usaully for porsche owners or by invite of a member of thier club. You can click on the names of the peple who run the even on the schedule to find out more about them.
You dont have to run the whole series of any club to participate, in the three races i've run, they've all be different series/clubs.
Most events you just get there early and register (like 7:30-8:30), some bigger (usaully 2 day) events require a pre register only.
All the events i been to they were extremely helpful with beginners, they'll even have a pro walk the course in the morning and tell you how he will attack each corner/slalom.
Mark