It seems like the group interviews change, depending on time, place, etc. I attended the group interview in Chicago last week. There were around 35 applicants there.
The process started with introductions from the SkyWest folks that were there (about 6, I think), and a short recruitment video. Resumes were collected at this time. That was followed by an explanation of the full interview process that we would undergo in SLC, if we were invited for a second interview. After that, they allowed us to sit and/or mingle with other candidates while they took people for a short one-on-one interview. Upon completing the one-on-one, you were told if you would be invited to interview in SLC, and then you are free to go.
I think that they are just looking to find some people with competitive experience and backgrounds to have come out for full interviews. We did not take any written tests. Everyone there had an opportunity to get a one-on-one interview.
My recommendation would be: 1) be there a little early (do not walk in late!); 2) be friendly and relaxed; 3) have confidence in your abilities.
Good Luck!