There was an old hack called vbHosting which was exactly doing this.
It was realy great and I would love to have it for 3.6 but it was forbidden and never updated anymore.
If I remember correctly its against the license agreement if users can create forums with own subforums and groups etc.
What you can do is create a forum for them and give a public group access to the forum. Make one of the users usergroupleader and they can decide who has access and who not on their own.
What the cannot do is create additional forums or subforums.