In todays online society where many people are just a name and a personality, friendships are occurring between groups of people that wouldn't usually cross paths.

Legally, you say there's nothing sexual, and he admins the group but doesn't seem to be harassing anyone from your description.

So you go off telling this persons friends that he's a pedophile, with no proof? And then you wonder why he's threatening to sue you?

I'm not a lawyer but quite frankly unless something sexual happens here you need to stop throwing such a serious accusation around. If something sexual is occurring between him and a minor then you can notify the police immediately, maybe a lawyer as well.