Thank you, that solved it for me
It wasn't completely logical though, if you wonder about me not understanding directly:
The first link only mentions activities, but my problem was with resorces, not activities - the grouping settings in my forums actually worked before I made the setting under "experimental", but not in resources like folders.
Anyway, your posts helped me (and by that also the 35 000 teachers & students that soon will be using moodle 2 at the Linnaeus University, Sweden
The moodle.org forums and the people helping out there are great!