How to create a testflight invitation code?

I am a registered iOS developer and a developer friend of mine is trying to send me a testflight version. For some reason he can't sent me an invitation - I believe this is because I am registered as a developer and used my only apple ID for the dev apple account registration.
Nevertheless, I am currently beta testing another app... So something weird is going on... Furthermore, I notice there's a "Redeem" button at the bottom of the testflight app so there should be a place to invite other users using invitation codes.

So how to send a testflight invitation code? If this is not possible, how can I resolve this issue?

after you add the user for testing. the user should get an email. open that email by your iOS device, then click "Start testing" it will bring you to testFlight to download the app directly. If you open that email via computer, and then click "Start testing" it will show you another page which have the instruction of how to install the app. and that invitation code is on the last line. those All upper case letters is the code.