In order to install an app using Testflight, you need:
a) An Apple ID
b) The Testflight app installed on your Apple device.
Next steps are:
1) The app's developer send you an invitation via iTunes Connect. The email contact is your Apple ID.
2) You have to accept this invitation so you could be associated to the developer account where is hosted the app to test.
3) The app's developer will associate your Apple ID to the Testflight app.
4) You will receive a notification on your device
5) Tap on the notification and then install the app