You can add userroles to an user or account by creating a page where you have the ability to select and and userroles with a user.
The Mendix administration module(which is probably in your project): https://appstore.home.mendix.com/link/app/23513/
Has a setup for this. If you enable acces for an admin to the account_overview page from the navigtion, you will be able to create and edit accounts as an admin.
Do you have the Administration module inside your project? There you have a administration map with the page Account_Edit.