I am a veteran of 35+ years of track experience, a NASA Instructor for 17 years, and worked Tech/Hot pits/Flagging at most of the the last 25 SAAC conventions.
No doubt you should START in the beginner's group. Learn the track, learn your car. Do NOT try to go out and kill it first session. You can always move up. You'll be amazed how much you have to learn, and starting in a faster run group will completely overwhelm you, and you will be dangerous.
Remember - you are taking out a Cobra on the track the first time you are going on track. These cars have earned the Widow-maker" reputation for a reason. If your car is set up for the street, it will be "twitchy", nervous, terrify you in fast corners, and most probably you will snap spin at some point. Learn the track and your car slowly, much for fun.
Why do I say all of this? I race a FFR Cobra. The track set-up is completely different from the street. I have seen Cobras spinning more often than any other car. I stopped instructing in Cobras because of the drivers determined to be Ken Miles the first time out. Be sensible.
SAAC run groups do not have instructors, and typically the drivers are not as keen to the rules of the track, than with other HPDE groups, such as NASA, Chin, HOD, and others. Be extra careful. Learn all the rules. Attend any and all classes, de-briefings, meetings. Learn passing rules, passing zones, and WATCH YOUR MIRRORS! (I may be that guy wanting to pass you, driving a rental car!)
I'll be there, not sure if I will have my FFR there. Let's talk if you have any questions.