When does the season usually begin? Also do you just join SCCA or do you have to participate with regional clubs (bmw, porsche)?

Check www.scca.org and see what they have in your area.

SCCA Solo II events in most areas are open to everyone, only a few regions require membership.

You can show up to BMW, Miata, Viper, etc. etc. events too if they're open to everyone-- they usually are, but it depends on the local club hosting the event.