Have a look at the administration module that is available in your app.
From there you can create the users and assign them the roles they need. Additionally you can invite mendix platform users to your app by inviting them via the development portal.
I feel kinda dumb, as I did not know there was an admin section in my app. Of course when I put admin in the search, it came up. I will play with that and see if I can get it to work.
If you get confused with module Administration, then replace it with module SystemManagement. SystemManagement is less confusing, has better entity definitions, better performance and a better logic.