You can add users to your private app in two ways:
1. If you know the email address for the Google account of a potential user you can add them to your app on the “User management” screen before sharing the app with them. They will then be automatically able to run the app on their devices.
2. Share your app with selected email contacts on the Publishing (from Studio) or Sharing (from installed app) screen. Go to the “User management” screen and you will see a user record has been added for each contact. Tap a user and select “New authorisation code”, followed by the “Email authorisation code” button.
Your contact will receive a user name and code which they must enter to gain access to your app. The email address of their user record will automatically be updated with their Google account address, giving them permanent future access.