I managed, though founding a bog in GJ, and one that we haven't still decided if it's GJ or K...
1- Groups created in backend don't create forums, must be created in frontend.
2- Seems permissions are not being given correctly, I'm owner of a group, can see the forum in kunena backend created, but I can't see it in kunena frontend.
Logged in phpmyadmin and saw kunena sessions table was not being updated, had access to all the other categories id except for that one...
So, fixed it by reseting sessions, writing an na in phpmyadmin...
Though I can't keep doing that for every group a user creates, have to investigate further...