Steps to follow:
Log in to the Google Play Store(on your mobile) with the test user account
Open the internal test link in the browser, make sure the browser where you open the Internal test link is also logged in with the same test user account
With one click of the link, an invite will be shared to join the program for internal testing
Accept the invite.
5. Click on the download it on Google Play
6. Click Install or Installed
7. A pop-up with device details will appear, select your device and the app will be installed on the device in a few mins.
8. The testers need to accept the invite then only the below internal beta app will be available for testing on the Play store