Hi Jurian,
You can not add $UserRole to $ProjectUserRoleList because they are different entities, right?
The MTA module comes with microflows to help you with the task you would like to achieve. First, make sure you set it up correctly as stated in the documentation https://appstore.home.mendix.com/link/app/80498/ . Use the snippets and microflows in the _USE_ME folder and check how the confirmation buttons on these pages are handling the system user roles and the tenant user roles.
Hope this helps :)