If the bunch is not part of a formal club and are a "turn at at XYZ at 6am and ride" then I think there's no point.

Basically there is probably NO ride leader as such. I belong to one of these ad-hoc bunches (but it sure doesn't ride that way) and sure there's a core group of people who normally say "let's go" etc but that's it. There's no come back or authority or anything.

However if they are a club that's different. As a club commitee member I know how hard it is to control/guide the faster riders. It's all rip, sh*t and bust there but you can have gentle reminders, a briefing before the ride etc. And of course in an extreme cases, cancellation of membership for the worst offenders.

Maybe instead of trying to tell off the bad riders you need to create something that rewards the good ones eg. an annual prize for the person who shows the safest attitude riding on the road. You could get some PR out of this by going to your local authority road safety and offer to support and run this and then get the media involved and have the public nominate- something like that, so showing that YOUR shop is all for positive riding and that YOU care about safety and other road users and in no way can be tarred with the bad cyclist brush.

And forget about the others-it's not worth it.