如何从电子邮件中添加TestFlight beta app邀请

I have test flight installed on my iOS 8.1 device (iPhone 5C), and an email invitation to join a beta release of an application on TestFlight. The email has the "Open in Test Flight >" button and the apple ID it was sent for matches the apple ID logged into TestFlight on the device. I have test flight installed on my iOS 8.1 devi