Ride safely: communicate, don't overlap wheels, call out or signal obstacles.

I ride on mid-level club rides, not fast training rides or hammerfests, where the "rules" usually vary according to the ride leader. That is, some ride leaders are more solicitous of weaker riders than others, who readily drop people without a second thought; sometimes, the entire group will stop with the rider with a mechanical, other times, a rider or two will stay to help, then re-group later.

On the rides I lead, we re-group at tops of hills and at turns when slower riders have dropped back out of sight. If someone is laboring and needs to shorten the ride, there's usually a volunteer or two to accompany them. Again, these "rules" seem best on the rides I do, where we can get less-experienced riders, who may not be safe on their own.