Cannot add an internal beta tester to Apple's TestFlight - what am I missing?

Question

I have enabled my app for TestFlight testing and invited a bunch of beta testers and successfully added them to my project. However I cannot add a single internal tester. The UI just shows me that I should select at least one user, however there are no users to select from.

Here are the steps I have taken so far:

• I am Admin in the Users & Roles
• I invited two internal testers.
• Both have confirmed the invitation and agreed.
• Both testers have the Technical role.
• For both testers TestFlight testing is enabled.
• Beta Tests are enabled on the uploaded build

This is what the UI looks for me:

Answer 1

Found the answer. The users are already added as external beta testers. Removing them from there, makes them show up on the internal tab. Not too obvious IMHO.