No TestFlight invite email gets sent for internal users

Question

I set up my application for TestFlight. For external users it is waiting for review. But if I want to invite internal users, I select them and press invite. iTunes Connects says

Answer 1

This is what worked for me in November 2017.

1. Get the build approved by Apple for beta.
2. Add the approved build in #1 to an app submission and save.
3. Now you should see the resend invitation buttons in TestFlight > Builds: iOS under the External Testers column, or TestFlight > Testers & Groups: All Testers under the Status column for each tester.

I'm not sure what triggered the invite email to be sent to internal testers, as there was only a button for external ones, but soon after I did #2, I got an invite as an internal tester.

Also, as I didn't track this process too carefully, I may have triggered the solution by changing the marketing build version in my app submission (#2) to be the same as my beta build version.