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

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: enter image description here

Solution

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.