I like to try and make my 'group sessions' as unstructured as possible. If there's a new person then I will teach them the basics a little, and maybe play a couple games (add on, follow the leader). I enjoy teaching people parkour, but I have unfortunately found that this causes roles to set in. i.e. I'm the teacher and they are the students. So basically instead of an open parkour jam it has become a "What does Brett want us to do?" sort of thing. That's not really what you asked, but I thought it might be helpful.
I've found the same thing.
Also, if you make the warm-up too hard, a lot of people just don't come back.
What I usually do is a slow to medium paced jog of around 400-800m (one or two laps around a track), then some dynamic stretching (Google it), then I'll ask people if there's anything they want to work on, or go into an open jam. I think it's good to start out with some specific technique work (do a bunch of vaults, or wall runs or whatever) before going straight into a free for all, but some people want structure and some people don't.
If you push too much in either direction some will give up. Of course, parkour isn't for those who give up so easily.
