The best way to get your answers is to go directly to the source, which the next good opportunity will be at Daytona on May 7 and 8. There will be many Spec Miata's running in a 90 minute enduro race then. Go to the race and ask the drivers, look at the cars and watch them race.
If you talk nice to the folks at registration, they may let you into the track at no charge or worst case, they will charge you $5.00. Just tell them that you are insterested in joining SCCA and would like to see the races.
Here is the link to the CFR SCCA website.
http://www.cfrscca.org
Here is the link to the schedule for the weekend, info as to where registration is and when registration is open,
http://www.cfrscca.org/forms/DayNatECR0505.pdf
Spec Miata is SM and is in Group 1. Their race is the last event on Saturday. Look for up to 80 cars to race in Group 1.
I'll be there as crew chief for a Toyota Formula Atlantic, car #81.