The groups really just depend on who signs up.
I have done it twice now. The first time, there was just one other family with two kids. It was wonderful with lots of attention. The second time, there were a lot of people. Maybe 5 to7 families most with multiple kids. That made it not quite so fun as there were so many people, who couldn't see some things well.
I'd avoid going on a Saturday. Most weekdays in off season are probably good. But it is always a crap shoot. But the odds are in your favor.