I have installed Testflight. When I attempt to open the Zwift app (using the link in the email I received), my phone just navigates to the Testflight page in the iTunes store. I don’t know why it’s doing this, since I’ve already installed Testflight.
Testflight invitation e-mails have trouble if you use the GMail app, or potentially other e-mail apps that are not the Mail app that ships with iOS 8. However, if you can open the e-mail from within Safari using a web interface for your mail, and click on the link from there, the link should work as intended.
After I got TestFlight installed, to get the link to work, I used Apple’s Safari web browser to read my invitation email (Yahoo webmail). When I clicked on the link in Chrome, it would open TestFlight but not Zwift.
I am using an older iPad2, and things are looking good so far! Here are some photos: