I have never found any group descriptions to provide more than a rough approximation, but that's in no way a negative statement. Groups should, I think, ride at the pace that best suits the group on any given day.
Except for beginner rides that really should coddle riders in every way possible, I think a group newcomer should expect to just give it a try and see what happens. If the group is too slow or fast or disciplined or undisciplined then just consider it a valuable learning experience.
No, I'm not cruel or heartless. I even ride sweep when needed.