How to invite Google Play Console internal testers

Here's what worked for me:

1. Go to "App releases"
2. then "internal test track"
3. then "manage Manage testers"
4. then click "expand"
5. the "Opt-in URL" appears as in the image below
6. send the link to all testers, they'll be able to "join the programme"

The system doesn't invite them, you have to give them the opt-in URL yourself. I think the email list is just for access control.

The Google Play Console Help details this step:

After clicking the opt-in link, your testers will get an explanation of what it means to be a tester and a link to opt in. Each tester needs to opt in using the link.