To add an app version to Test flight you will first need to create a Debug & Release version in Zapp.
Once a release version is created it will automatically show up in Test flight.
To access test flight - Open iTunes connect, choose your app and click on the Test Flight tab.
You will see the last version you created on the top.
A user must be added as an internal tester to download the version.
Click on iTunes Connect Users
You can search for existing users on the account to add them as testers or go to Users and Roles to add users to the account.
Once a new version is uploaded to the account, all testers will recieve a mail notifying them of the update and providing a download link.
Users can also enter their installed Test flight app to see all versions.