The OP's group sounds pretty ad hoc. Formal clubs tend to have well-defined ride descriptions with a designated, experienced ride leader whose responsibility it is to lead the group according to the ride description. Every club does, however, have a different way of listing rides. On club defines the ride speed as the speed the participants can maintain on the flats. Another club defines their A,B,C, etc. categories based on the time over a standard loop ride. A third club defines the groups in terms of a repeatable time on a specified climb (and provides a table of average speed versus climbing per mile). Some leaders ride at the front, some lead from the rear.

It's not surprising that this ad hoc ride has chased away other participants. It's a shame the "leaders" of this ride can't see their way to splitting the ride as a means to provide a better experience for all.