Depends. There are three typical categories of Autism groups:
1) Autistic-run
These meetings tend to be casual, for the purpose of meeting other autistics and having fun talking. They may or may not talk primarily about their autism. Or it will come and go with conversation. These tend to be fun, and the discussion isn't rigid at all.
It tends to only be high-functioning people at these meetings.
2) Parent-run
These meetings also tend to be casual, but the difference is the (sometimes overbearing) presence of NT parents. While an autistic-run meeting might only have one NT parent or so, it's not uncommon for NT parents to make up well over 50% of the membership of a parent-run group. Even in the meetings designed for autistic people, many NT parents decide to tag along, drag their child there, or just go alone.
I say "drag along" because many of the actual autistic people don't actually want to be there. Many of them are on the lower-functioning side, and would rather stay at home than go out. On the flip side, the general atmosphere of parent-run groups tends to turn off higher-functioning people. Many "free-range aspies" are looked at with disbelief by the parents, because they believe that people with AS/HFA aren't capable of being there without their parents.
3) Psychologist/Doctor-run
These tend to play more like group therapy sessions. There is a set facilitator, and a set topic to talk about. It generally goes like a round table, where people speak their mind about that specific topic. Functioning levels could vary. It depends on whether or not the autistic people are looking for help with anxiety, social skills, depression, employment, you name it.