I agree with knowing there is a group at my level. There are few beginner/C level rides out there and I won't join a B ride knowing I will hold them back. (And I'm sure there are plenty of ladies looking for super fast paced rides.)
Provide clear definitions of each group's pace.
Time of day is also important to me, but you'll never be able to accommodate everyone. I can't ride until after work (5pm ish)
Short clinics associated with the group would also be nice (rules of the road/group rides, basic mechanics, nutrition, building speed, climbing hills, etc).