A Group is for the dudes that have all the required safety equipment,meaning firesuit, shoes,Gloves, appropriate roll cage,ect. These are the guys that have no issues making it around the track and are working on getting closer to other cars and running a better, faster line. Just because you have a roll cage does not put you in A group.
B Group dudes are the ones who are making it through the course without issues,i.e. not spinning ,not understeering. They are running a good line, maybe not the best line but they are learning.They are starting to go faster and try new things.They may or may not have the right safety equipment to tandem but their skill set is not up to the level just yet.
C Group dudes are the New guys ,the guys still learning their cars, learning the track,etc. This is where everyone starts.There's nothing wrong with being in C group, we were all there. These are the guys working to link the course and start understanding what the right line is. They are figuring out what they need to do in the car to make it work.