Here is the overview from the Docs.
Invite a tester to a distribution group using their email address. If you don't have an existing distribution group, you'll first need to create one. The tester will be notified that they have been invited to test your app.
Distribute a new release to the group.
The tester will be notified that a new release is available. The user will try to install and will not be able to
Navigate to Distribute > Groups > Select the Group > Devices
See the user in the list and click on the menu item "Copy UDID"
6. Then follow the process of adding the UDID to your provisioning profile as described in the documents linked in the docs.