I have just added a new private user group and realised that there was no way anyone would know about this unless I explicitly invited or mentioned it!
How about a wiki topic that can be used to list known user groups, either here or elsewhere?
ClarionHub private user groups:
(send me a PM @brahn to get invited!)
Other user groups
I don’t actually have any yet but perhaps the distributor in your local region could put you in touch? Make sure you come back here and update the list if you do find any!