"C" ride?
Just about the only problem is riders that dart far ahead or drop way back. Both are annoying for the ride leaders that are trying to set a good pace and not lose anyone.

"B" rides or higher?
Pedal...coast. Pedal...coast. Some new riders that are drafting tend to look just at the wheel right in front of them. So when a slight gap forms, they pedal too hard, roll up too close, then coast back. Over and over. That's hard on the riders behind. And it's rare that anyone tells the new rider.