A group Z2 ride is a utopian place where theory races against practice - almost nobody in the group knows their lactate thresholds, but they can explain to a friend in detail what pace to ride at the front of the group.
So when you turn up to someone's "Z2" ride, don't expect it to be in your Z2. First of all, there is more or less a theoretical consensus - Z2 sessions should be just below LT1 (the first (aerobic) lactate threshold), and they are a great tool for developing aerobic capacity. But even with that definition agreed, keep in mind that abilities differ, units of measurement differ and, of course, the methods of setting zones differ.