Our group is quite a bit larger than yours, over 100 on the Google Group list. As others have said, more than 1 ride leader. We're big enough now that we're not looking for more riders. But back when we were, we used to advertise. Then we needed to be able to deal with whoever showed up. We used several different methods. One was appoint 3 ride leaders from among us, ahead of time, for routes that were determined ahead of time and cue sheets written. Then we'd start off up a long "sorting hill," with a prearranged regroup area at the top. When a ride leader judged that those who should be with their group had arrived, he or she announced that they were leaving and did so. Each group might complete the same route or use a shortcut, as determined by its leader. The leader appointed a sweep from among the group, usually one of the faster riders.
The leader would regroup at known points. Anyone could pass the leader and go off on their own, maybe getting lost or maybe not.
Though this was not common knowledge, those who were added to the list were those who rode well and cooperatively and who "came inside" for beer or whatever at the end of the ride for social hour. Our group is almost 20 years old. We ride from 40-100 miles. On shorter, pouring rain rides we may not stop. On all others, there's a coffee stop, though never a full lunch stop. We don't believe in eating a lot in the middle of a ride.
If the groups were all going to ride the same routes, the fast leader might direct a rider who couldn't keep up to wait for the next group. Every now an then it would become necessary to give a rider a cue sheet and tell them to break off from the group. It's not possible to do this perfectly. There were frequently riders who started with us, but then as some point we simply never saw them again. We figured they fell prey to some large carnivore and drank to them at the end.
As we've aged, we've acquired younger riders to carry on, though so far we've all been riding so long that the younger set are not all that much, if any, faster than we oldsters are. We don't attract racer boys or girls any more, though we used to. They have their own groups and venues. Our ages are about 40-70. When people simply can't keep up any more, they retire from the group. Sad, but it happens to all of us.