The speed in group 1 will be slower and (correct me if I'm wrong) has numerous restrictions to overtaking so there will be opportunity for more cars to be in this group.
The instructors also make a lot of difference to fresh track drivers as the more important thing is the racing lines and braking points. Safety first, and there will be numerous marshalls on duty all over the track manning flag stations.
Highly recommended if you're new.
I don't really understand the comment about Group 2 having lots of turtles. :hmmmm Previous experience in Group 2 saw a couple of quite quick drivers/cars as well.
Just compared to Group 3, of course all the seasoned kakis are there.
Go according to your comfort level. You're out on the track to have fun with yourself and your car, this is not a competition event. ::cheers:
p/s Hi Alvin, Andy here. Got in on time this time.