Once people(Tester) accepts the invitation they get a mail from testflight with subject Get Started. If invitees click on the get started and completes all the required step successfully you will get a auto generated email from Testflight with subject line XYZ added a device and the body of the mail contains the following details.
Then you can add this device id in provisioning profile manually. The only thing is "invitees must use his/her mobile device for opening all the mail from testflight".