Dear Andi, you have 2 options:

1. Manually adding users via DB ( which won't really be a webinterface solution).
Not recommended.

2. Get a plugin that will allow you to log in as needed user and simply join needed group while being loged in as that user: http://www.oxwall.org/store/search/plugin?q=login

Note: since those are plugins developed by third party developers we do not know well they work. Make sure read reviews and forum posts before ordering any plugin.

Oh sorry... i meant something different... I mean the User Roles... Like registered etc. You know what i mean? ^^

@Andi , you mean you would like to add certain users to certain user role?
If yes, you can visit needed user's profile as site admin >> click "Moderation" >> click "Change role" >> assign desired role >> save changes.

Hope that this helps. Let me know if you meant something else.

Well as an Administrator ther are no Modules to use when I'm on one persons profile.

Go to the persons profile as site admin and you can change rolls 

Thanks. Worked like a charm :)

Cool