BB. We don't have groups as this software is not supposed to support the large amount of categorized forums.
However, what you describe, you could achieve using 'Super-Title' feature
. Login as admin and go to your forums management section. Edit some forum, and the forums list below (preview). From there note the following:
Supertitles appear BEFORE main forum title, emulating category, and you probably may add supertitles AFTER you created a list of forums and arranged them in desired order. Go to forums preview when editing any forum, click on forum's title when you want to put supertitle, and add it in corresponding field. Notice that moving the main forum title affects also supertitle, so putting one forum from one "category" to another means actually that you need to remove supertitle from this forum and add it to another forum's title. You may use straight HTML in supertitles.