Inviting the user to App Store Connect
1. Go to the App Store Connect and click on Users and Access.
2. Add a new user to your account by clicking on the add-button (+)
3. Enter the user's name and email in the pop-up and click on Invite
4. Important: Make sure to use the email of the main account on the test-device that will be used
Inviting the user to TestFlight
1. Go to the Apps section and click on TestFlight
2. Go to Internal Group > App Store Connect Users and click on the add-button (+)
3. In the pop-up, select the users you wish to invite by ticking the box
4. Finally, click Add
Internal Groups and External Groups
When testers are invited to beta-test your app, they are prompted to install the TestFlight app from the App Store on their iPhone or iPad, if they don't have it installed already.
Internal Group testers will be notified every time a new build has been uploaded.
If you do not wish to have the users notified every time a new build has been added, then you can create an External Group. External Group testers will only get access to the builds that have been assigned to the given group, and only when the builds have passed the beta-review process. External testers will therefore not be able to get access to new builds immediately, but a few days after internal testers.