I think most people who use forums these days would understand that you get restricted (if any) content until they sign up.
I can't say I have run a forum before, but I remember the days when I ran my BBS....Trouble-makers are less likely to leave stupid/abusive messages if they have to go through the "bother" of signing up.
Restricting access to a few groups means the "I hate Opel" people can be less of a pain.