I'm not part of those organizations but I have a few friends that do autocross with the SCCA. As far as I can tell, they enjoy the events. You can probably look up some events and go to one to see if you like it. I think you can still participate as a non-member, you just have to pay more. Not sure about this though. You can check their websites for details.
1988 GT - Stock
*SOLD* 2012 GT Premium, Black on Black, 3.55 Axle, HID and Security Package, Pypes Pype Bomb Axle Backs