I don't think it's possible. Once the TestFlight invitation is accepted, it becomes associated with the Apple ID that accepted the invitation. That association allows the app to be installed on any other devices using the same Apple ID, without needing to accept an invitation for each device. I wouldn't be surprised if the limitation is by design, to prevent people added to a private beta (not specific to Gospel Library, but any beta) from being able to forward the beta to large mailing lists.
Samuel Bradshaw • If you desire to serve God, you are called to the work.
