When people (tester) accept the invitation, they receive mail from testflight with the topic Getting Started. . If invited users click on the start and complete all the necessary steps, you will receive an automatically generated email from Testflight with the subject XYZ added device , and the body of the message contains the following data.

You can then add this device identifier to the provisioning profile manually. The only thing is that "invitees must use their mobile device to open all mail from testflight."
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