Currently, individuals can only be assigned as Use Case Testers if they are testing in their primary org in AMP. They cannot be assigned as testers even if they belong to the org if it is not their primary org. In order to assign users as testers in a workaround, Admins have to move individual accounts from org to org. This is inefficient and frustrating especially if the person works across various teams/assets/products and there are many orgs they're associated with.
A solution would be the ability to assign a user as a use case tester to any org (regardless of primary org location).