The only path riding I have done has been in other countries. I have found that saying/yelling "bike left, passing" and also saying thank you helps. I slow down, but it requires a lot of attention.
Two weeks ago I went on a club ride in the middle of the week. There were 50 people and the last 8 miles of the ride were on the Nashua Rver Rail Trail. I have passed this trail on many of my road rides and it is always filled with people on the weekends. It was fairly open at noon on a Wednesday, but there were 2-3 slow riders and one little kid who we told "Just stay there ujntil we pass." I was at the front, with about 8 people and we were flying, going about 20-22. The group was really spread out and some people didn't get back for an hour after we did, but I don't think as a leader I would have taken a group on a path. Just too many things could happen.