A group of serious roadies -- not just 12-15 mph Freds riding road bikes and wearing the garb, but serious roadies -- would have dropped you easily. Sounds like you may have been caught up in a loose knit group of casual riders who just figured you were part of the gang.

Maybe they didn't know you weren't part of their group. The only group I ride with is very casual with a mix of bike types, clothing styles, etc. When we have a dozen or more riders I wouldn't know whether we'd picked up another group member somewhere along the way, or just caught up with another casual rider or two. Our average speed is only 12 mph so it would be easy for another casual rider to join and keep up with us, and we probably wouldn't even notice.

That applies as well to most of the charity, event and similar casual group rides in our area, which can sometimes involve dozens of riders.

If you'd rather extricate yourself, just carefully move to the right, signal others to pass with your left hand, and gradually slow down until they're forced to pass you on the left. If you need to stop use the usual "stop" hand signal (left forearm downward and away from body, open palm facing rearward). Incidentally, while I see some cyclists signalling for a right turn or move toward the right by pointing with the right hand, I prefer the old fashioned all-left-hand signals. For one thing, this leaves the right hand free to operate the rear brake if necessary. Using the right hand to signal, while suddenly needing to squeeze the front brake lever with the left hand, can be disastrous. Good way to get ones face acquainted with the road. (Assuming the bike is set up with right=rear brake, left=front brake -- apparently customs differ.)