Hi there
Have you by any chance already experimented a bit with the Administration module within Mendix? With some tweaks I suspect that it might solve all your administration & user rights problems!
Here, have a look at the documentation: https://docs.mendix.com/appstore/modules/administration/
Good luck
As Juriaan says, take a look at the Administration module. It should already be in your app.
If you want to do this outside the Administration module take a look in the System module. You can assign roles by setting or removing the appropriate UserRoles associations between User and UserRole objects.
Good luck!