I hate to sound like the man but there is a good reason, namely road rage. Stop or yield as you would when your alone and then catch up to the group later. If the road captain sees that the group has been separated he will either slow down or pull over at a safe location. A good technique of getting the group together without interrupting the flow is for the end group to slow down and let the front group get onto the road to continue the ride. When they turn yellow stop if it can be done safely.
Same thing applies to bikes.
What a concept. Not so, one of instant gratification. There are old bikers and there are bold bikers but there are no old bold bikers. Keep the sunny side up.