is there a possibilty to create i.e. 10 different forums within XF, but for each forum only 1 specific usergroup has permission to see it and/or use it, the others not?
Click Users -> Node Permissions - > Choose a node (click Permissions) -> Then click on the group you want to have access and set the appropriate permissions.
But does that exclude all the other usergroups?
I think you click Permissions beside the forum, then click Private Node at the top, THEN tick the usergroup you want to grant access to. Plus checking their permissions.