I did a group ride this past weekend that was reminiscent of this thread, except with 80 people there were B, C, D and who knows what groups. The A guys disappeared pretty quickly; I think I ended up in the more or less B group. We finished with a 17.0 mph average, which is roughly where my solo rides end up; in a nice paceline that was occasionally being pulled by a tandem, we cruised for several miles at 25+. It was my first really big group ride; it's been a long time since I've had that much fun with pants on.

Pics are here. I'm the ape mid-corner in picture 122...

did