Okay af3, this requires manipulating your forums in the bbPress backend.
For me, new group forums were being created as a child of forum ID #1.
So what I did was change forum ID #1 to a forum category, then I moved all existing group forums under forum #1. Lastly, in step #3 above, I changed the ID from 3 to 1.
Now all new BP group forums are created under forum category ID #1.
Report back and let me know if this works for you.