Your friend will have potentially filled out the form saying he/she happy to test now, hence he/she getting an invite.
As an investor you will get ‘first dibs’ when the public rollout commences. If you signed the form expressing interest to test this preview rollout like your friend must have, you would potentially (no guarantees) got an invite.
I hope that makes sense.
This might help clarify it further, although only a snippet. It may be worth reading the thread in its entirity to get full context: