Beta testers cannot access Apple ID emails.

How can an app tester install a beta app onto their device when the Test Flight invite goes to the email associated with their Apple ID, and that email cannot be accessed on the device that we want to use for testing?

I am distributing a test app on behalf of a client for their staff to test, but many of the testers have an Apple ID set up using an email they only log into from a desktop, and either they don't know how to log into to it on their mobile device, or it's a work email that can't be accessed remotely, etc.

With the update to iTunes 12.7 we are no longer distributing .ipa files for upload with UDIDs to the App manager, but if a tester can't access the link for test flight from the device, is there a workaround to still allow the beta app to be installed?