I have never seen enforcement of a limit of number of people on the summit. My theory has been that If I dont want to see crowds, I make sure I am the summit before 11 AM or after 2PM.
Lincoln Woods used to be famous for enforcing the group of ten rule and the outfiltter/guide permit. When I was working with the local boy scouts many years ago, we got asked to show them our permit 2 years in a row at the bridge. I know of a few groups that were turned away due to group size or lack of permit. On another trip we were shadowed by a ranger all of the way to the Bondcliff junction late one afternoon. He waited until we took a break at the junction and then sped up to us to make sure that we werent camping at the closed off area (we werent planning to).
I have talked to couple of forest service folks over the years and limiting group sizes is very low on their priority list. They get bad PR and the fines rarely are enforced if challenged. The primary group they go after are commerical entities, and most of the time, when a commercial entitity applies for an outfitter guide permit, they put conditions on the permit to only allow small groups and limit where they can go (used to be evans notch, the kilkennys or the area south of the Kanc between chorcura and the Osceolas but I expect that has changed with the recent wilderness designations). THe llama bus that used to work Evans notch operated under one of those restrictions.
The hassle is defining a group, Is a church group anymore or less than a VFTT group? Is a meetup group more or less than a VFTT group?. Are a group of "friends" on facebook any more or less a group. Sure when a summer camp trucks 40 campers over to a mountain with paid staff members its cut and dried. Normally on popular summits, I dont think groups are really contributing to the crowds, there are far more unaffiliated day hikers than groups.
When doing a presi traverse this summer with a meetup group (less than 10) we played tag with an annual AMC presi traverse group which was closer to 20. They had two leaders and two coleaders but hiked as group until the end of the day when they stretched out.
As Bob and Geri pointed out, Meetup groups can get very large on occasion and are subject to derision by some. Their leaders are sometimes a bit clueless and some are fairly new to the woods and group sizes can be an issue. (the same could be said of VFTT groups long ago) On the other hand they are getting a lot of folks out into the woods that normally may not be and given the need for advocacy to keep funding intact for national forests, its probably not a bad thing to expand the user base or the number of trails being closed will just keep increasing. I find that most meetup leaders eventually figure out that smaller groups sizes work better. I also see that some of the meetup groups are doing trail maintenance and having wilderness first aid courses and members are going into other organizations so the net result is probably a benefit to the outdoor community.
Heck, I suppose the WMNF can adopt the Baxter State Park Rules and that will definitely keep the group sizes down.