I've been in exactly one group ride that was run as it should be (well, two, both the same group though, two different days/routes).
Both had a leader who was extremely well respected locally, took no **** on the road from anyone, checked everyone's ego, and asked people not to come back who couldn't obey the ride rules.
You have to have a leader willing to shut **** down and get rid of the bad apples, or it will devolve and the good people will stop coming because it's dangerous/annoying.