Force all users into a secondary group
 

I'm looking for a way to force all users into a secondary usergroup. I thought there was a built in function for that, but I can't find it. I don't want their primary to change, so I can't use the Prune / Move Users.

Basically I want to give everyone access to a certain forum by default, but allow them to opt out at any time. By forcing them all to a user-joinable group, this would work.

Any ideas on how to do this??

Basically you want to have them sign up on the site and automatically also automatically become a secondary member of a usergroup? I think you'd have to write a plugin that hooks into one of the registration hooks and add it then.