I have led a adult group ride for my cycling club www.mainecoastcycling.com for several years and wanted to share some of my thoughts. Some of them may be applicable for group rides of younger folks.
A group ride leader is part Nascar race spotter and part cowpoke on the trail. During a Nascar race, a spotter will sit at the top of the grandstand with a view of the track. The spotter is in contact with his or her driver and will be communicating constantly with the driver. You can hear "Wreck in turn 3, go low", or "Junior's on the inside!" coming over the radio during a race.
I will sometimes give out warnings like "watch out for car doors" while riding along a beach in the summer, or "there's usually sand on the road coming up" to give a heads up to my group. I try to focus further down the road than the rest of the group to spot problems as soon as possible.
In the old westerns, the cowpoke on the trail is watching the herd, looking for strays. While my riders don't usually stampede (except for the coffee at the end of the ride) I usually will try to keep track of the cyclists. Every ride will have cyclists of different abilities and mindsets. I will usually announce a drop policy which can vary. I might issue a "no drop policy" if its a ride on an unfamiliar route. If a rider is off the back, I'll ask the riders in front to soft pedal until everyone is riding together. If the route is familiar, and especially if its hilly, I will give locations where everone can get back together, usually at the end of a road. Hilly roads are almost impossible to keep riders together.
When a ride starts, I'll get a count of the number of riders. I'll also look for new faces and in the first few miles of the ride try to guage their fitness (I'm not sure if rider profiling is legal, but...) to figure out if they will be scooting off the front, hanging in the pack or out off the back. If a new rider is off the back, if no one else is back with him or her, I will drop back and help them catch up, or give directions back to the start if the rider is not up for the group ride. Younger riders will need an escort if they have to turn back.
Riders off the front can be troublesome too, often going all out in the wrong direction. I may be able to "bridge the gap" and reign them in, but often I will have to let them figure it out for themselves. Young riders should be able to be chased down fairly easily, but I would recommend not giving them much of a lead.
This was an excercise mostly in blog posting, but I hope folks will give some thought to the dynamics of a group ride as we get back on the roads this spring.