I\'m having trouble with setting up a beta test with internal testers using iTunes Connect and TestFlight.
I\'ve added users as internal testers inside iTunes Connect,
Its the issue with Gmail definitely.
Forward the invitation mail to one of your mailID configured on your Mail app on iOS device.
Open the link now from your apple mail app.
The link works now.
In response to @haupman's issue.
It seems that trying to install, when accepting the invite in the gmail app, on an iOS device, won't always work.
The workaround was to forward the email to an address that is synched to the iOS mail app and then click on the links from inside.
As Fluffhead said, Gmail app won't work, since it opens Chrome for the link, which has no use.
Two options to accept the internal tester invitation in a mail from iTunes:
1) Use iOS Mail to click the link in the mail, then iOS Mail will open Safari automatically.
2) If iOS Mail is not available, copy the link and paste it to Safari. For example, you can use Gmail to click the link, copy the link in Chrome, and then paste it to Safari.