I got my app approved for distribution through the new Apple TestFlight app. I tried to invite an "internal tester" through iTunes Connect ->Prerelease ->Internal Testers
but all I see are 2 info boxes:
To start testing, select at least one Internal Tester.
Internal testers can be added in Users and Roles. Internal testers are iTunes Connect users with the Admin or Technical role.
I already added an Admin an Technical user to Users and Roles
but I can't invite them.
How to invite an internal tester?
Problem solved: The email for an internal tester was also used for an external tester. As soon as I removed the email from the external testers list I could invite the user as an internal tester.