I have no personal experience with meet-up hiking - I generally prefer to go solo or with a small group of people I know and trust - but I had an experience a few years ago in Yosemite on Mt. Dana that blew my mind. My family and I were about halfway up and took what was planned to be a short break. A few minutes later a stream of hikers began to go by - literally dozens. Our break went much longer than planned because there was really no way to jump back on the trail without literally merging.

We watched this in growing fascination and finally saw an opportunity to get on the trail after maybe 60 folks had gone by. We discovered they were a meet-up group out of SoCal and were training for - you guessed it - Mt. Whitney. On the descent we were ahead of this main group and were still running into a people on ascent who asked if we had seen a group of meet-up hikers along the way. These folks were easily 3 hours behind that first main group, and clearly struggling. I never did see anyone who I could identify as a sweep.

I remember looking at my wife and we said the same thing simultaneously - "Like herding cats."