Adding user and their details IS exactly what I want to do as well!
But if you tell me that I still won't be any closer. There is probably a simple button or set of fields in the IO dashboard for doing this so why are you keeping it a secret??
Make sure that your app is associated with a group, and that the group is configured for before-download authentication, and that the app has been published. Then go to the app's url. It'll redirect you to the user's registration/login page.
Alternatively you can register users from within the app. Again you need a group, configured for in-the-app authentication. Then you can say...