DH and I do 2 bigger rides a year and both are show and go. Otherwise, the rides we lead are screened by me for the person's appropriateness for the ride. After doing the larger rides for a couple of years (sometimes there are 40-60 people on these rides) we got at least 2 more leaders involved and split the group in 2 at the start. DH and I lead the faster group (not fast by most club standards), with an average of 14-ish. So, as sweep, I end up with people averaging 13-ish, sometimes we are tighter together. The slower group averages 10-12, but one year, we had a real new rider (she had just learned to ride) who ended up coming in with the sweep at an average of 8. So, DH and I leave first and we tell the leader of the second group to wait 5 minutes and go. It's the same route, so if someone at the back, with me, and is struggling, I just have them pull over and wait for the next group. You could do something like that. Also, the other club I ride with tries to keep no more than 8 people together in a group, leaving a gap between groups. We've had some complaints and the police called on us when we are riding in large groups, mostly because of bad behavior of riders.
When I lead for AMC, we are required to keep our group together and have a sweep to meet liability rules. The rules are based on hiking, but we have adapted. We just tell someone who wants to go ahead they are free to go, but we have no responsibility for their "care," if they do.