I've seen groups like that. They not only assume cars have to stop, but also other bikes and pedestrians as well. Just because there a lot of people traveling together does not mean they gain some kind of magical right to block others. I ran into a situation like that a while back. Riding along the bike path on my way back from a 5 hour ride I see a large group of bikes with a few dudes blowing whistles to organize the whole thing.

They cross the street and then sit in this large group and decide to head off into the bike path. With oncoming bike traffic they just move a bunch of bikes into the center to block anyone else and assume they have some right to do so. This was a problem since I was traveling at 35km/h and had to make an emergency stop or take them out - a good thing I had disc brakes.

These guys aren't traffic officers and have to wait like anyone else - especially when there is moving traffic on the path. If the group gets broken up then it has to regroup later on when it is safe to do so. I for one really dislike this kind of behavior. Why should everyone else have to wait 5 minutes for them?