Users can can multiple roles at the same time, allowing very granular access to your resources and/or managing ApiOpenStudio.
To manage and assign roles to users, click on "User Roles" in the main menu:
Here, you can view, add or remove roles to users.
Click on the Plus icon:
A modal will pop up:
Select the user from the dropdown (there is a ticket to update this to autocomplete, since this could potentially get very long), select the role that you want to give them, and to which account/application.
If you are giving a user administrator role access, this role does not need an account or application, because it is global.
If you are giving a user Account Manager role access, this role does not need an application, because it is applies to the account and limited access to all applications under that account.
Click on the delete icon (trashcan) for that user role: