I see those groups on the local MUP all the time on weekends if I go out later than @9AM. Crazy to do that on a weekend with so many casual riders who probably don't clearly hear 'On your left', don't know what it means, or they are wearing earbuds and don't hear anything at all, and while going w-a-y faster than the average MUP user. It sounds like you weren't doing anything wrong by riding in the middle of the right side of the path.
Idea for some fun: If you see it all the time, why not bring a small checkered flag on your rides and pull it out and wave it when you see a group coming towards you.
