I am an assertive club rider. I often lead beginner rides or take the back while another leader takes the front, and keep the group together, in straight lines, even if that means firmly giving instructions to someone who's endangering the group. I mostly do that with new riders. If all are experienced, my comments will take a different tone, more like 'Come on girls we can do better.' I also make sure most important safety aspects are covered before we leave, and chat about the others on the road. Sometimes I apologize after the fact if I barked something in a manner that was rude. But they quickly understand that safety is at play and usually they thank me.

I'm a pacifist, but I think I would have ripped off those earphones from that guy, and given him a good lesson on group safety (sorry this is NOT just etiquette). This is the most absurd thing I've ever read about group riding. Riding as a group is very dangerous and everyone's behaviour impacts all of the others' safety.

Please, please, do something, promptly. Tell him yourself, or seek the help of someone you feel has authority over the group. This is completely unacceptable. I don't want you to be involved in a crash because of such absurd behaviour.