I went to two of these after early On the 20th Century previews. It is after the performance. Members of the audience who want to attend just stay while everyone else leaves and the participants come out and sit on chairs at the front of the stage. Some questions were accepted from the audience members who stayed. No signing or informal contact with the audience.
Of course with this show the suspense was whether Kristin Chenoweth would attend. She didn't show up at either and in retrospect this should have been no surprise since she had a self-imposed ban on loud talking during the show's run. From what I read here she was gracious and cooperative with those who stage-doored, but she seldom spoke in anything but a whisper.
I really don't remember anything very clearly about who was there and what was said. At the first, Peter Gallagher was out of that preview with the flu or something. His understudy James Moye joined in along with Andy Karl and the porters and the director. I don't even recall if Peter Gallagher joined for the second one, (although in the two post-preview shows I saw I really liked him and didn't think he was getting enough recognition). I think that the only females to attend were one or two of the chorus ladies who talked about what it was like being in a hit show.
So I guess you can tell that I didn't find them memorable, since I don't remember them, and could have easily done without them. But it is a crap shoot. If Kristin and Peter and Andy and Mary Louise had come out and bantered with each other and played to the audience then it could have been a lot of fun. Anyway, all you have to do is hang around for a short while after curtain call and see who comes out on the stage.