I can't add testers to iTunes Connect: Users and Roles → TestFlight Beta Testers → Internal

In my case, this happened because I was set up with the Technical role in the iTunes Connect team.

Users with the Technical role are not authorised to add internal beta testers.

You need to find someone within the team who has either the Admin or Legal role in iTunes connect.

This person can then either:

• Set up testers themself.
• Set you up to have the Admin role, at which point, you will be able to add testers yourself.

Without the Admin or Legal role, you will not be able to set up testers.

The UI does not seem to have any feedback to inform you of this, which is unfortunate.

Apple makes this process painfully un-obvious, but here are some steps that may help:

• Login to iTunes Connect and navigate to "Users & Roles > iTunes Connect Users"
• Click the + icon near the top to add a new user
• ADD THEM TO THE "TECHNICAL" ROLE!! This is the important part Apple fails to explain. If a user isn't assigned to the "Technical" role, their name won't appear in the list of users to select from under the "Users & Roles > TestFlight Beta Testers" section.
• The new user will receive an email invite. Prior to them accepting, you will not see their name under your list of users. Once they accept, their name will be listed.

• Navigate to "Users & Roles > TestFlight Beta Testers" check the box next to their name and click Save. -OR- Navigate back "Users & Roles > iTunes Connect Users" click on the new user's name and in the upper right corner, switch the "Internal Tester" toggle to ON.

• Navigate to "My Apps > (app name) > Pre-release > Internal Testers" check the box next to the new user's name and click "Invite"

NOTE: I could be wrong about this next part but in my experience, the new user will not receive an invite for a previous build. You must publish a new build after the new user is added. Only then will they receive the invite.

As of late 2015 / early 2016 Apple has started to decommission the Technical role and replace it with the App Manager role. A new role of Developer was created, which also allows internal testing through TestFlight, while having somewhat less privileges. Unfortunately the hint text on the TestFlight panel is still not updated and lists only the defunct Technical role besides Admin and Legal, causing a lot of confusion.