Since I've seen my question asked before, I'm going to post how I got around this problem in case anyone needs it in the future.
Users and User Groups work just like any other content object (like an article) you don't need to make them using user/register. If you set up a button that sends the class id & node id it will create a new user in that node. This will, however, disable the email checking (which is okay for me, I was trying to disable it anyway)
You can use the content create toolbar in administration. http://ez.no/ez_publish/documentation/customization/components/toolbars/how_to_customise_toolbar
But I don't know, if anybody can make a subtree limitation in this user group when creating this new user automatically. Target is: One Folder, one user.
