Why not go a little further and allow ‘group threads’ where users can choose who can (or who cannot) see and reply, along with a category setting which allows or disallows them. Mods and admin always see them (or not, if specified in the cat settings).
These could be used to organise meets, campaigns, competitions, or just for social purposes. Having them in categories make them easier to find and categorise.
They could also be used as a replacement for PMs - ideal for sites where PM abuse is systemic.