title: How to create a testflight invitation code?
body: <p>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.<br>
  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. </p>
  
  <p>So how to send a testflight invitation code?
  If this is not possible, how can I resolve this issue?</p>