TestFlight is now part of iTunes Connect
To use TestFlight, you’ll need to be a member of the iOS Developer Program. You can upload your app binaries and import a CSV file of testers into iTunes Connect. While apps built with the TestFlight SDK will continue to function, we recommend that you remove the TestFlight SDK from future releases.
When a developer invites you to test a beta iOS 8 app, you will receive an invitation email with a link to the beta app and the TestFlight app on the App Store. Once you download the developer’s beta app and the TestFlight app, you’ll be ready to give feedback.