Cannot seem to add internal testers with existing iTunes Connect accounts

I am trying to add internal beta testers to my app. The app is still in development, so I do not wish to submit the app for review for external testers. It seems the only way I can add an internal tester is if I add them as a new iTunes Connect User. When I try to though, I cannot because the Apple ID (email) of the person I want to invite already has an iTunes Connect account. Is there any way I can add people who already have Connect accounts as internal testers?

Thanks in advance.