This will work, although it may return more than what you expected. Each user can have a different role for each app, so you'll see duplicate user ids in the results.
GET http://localhost:8888/api/v2/system/role?related=user_by_user_to_app_to_role
This will work, although it may return more than what you expected. Each user can have a different role for each app, so you'll see duplicate user ids in the results.
GET http://localhost:8888/api/v2/system/role?related=user_by_user_to_app_to_role