basically what i did was go to https://club.ubisoft.com and then to -> friends. but before that i used a community key and entered it on the beta redeem on the thecrew-game.ubisoft.com/ webite.
for some reason it said that none of my friends had a ubisoft account. So i typed the username in the search bar right at the top. I sent my friend a friend request and he accepted it. and then he sent me a friend request and i accepted him (i don't know if it was an error or it was supposed to be like that).
Anyway I went to the CLOSED BETA tab and now i could invite my friend. i invited him and he got an email that I sent him an invitation. But the key wasn't in the mail and on the website. He got another mail 1 hour later with the key.
It was a hassle to get my friend invited but eventually it worked.
I first had him make a Ubisoft Club Account even though he already had one, Had to search him by Name instead of through PS4 contacts.
After he accepted I could send him an invite for the crew beta, He first got an email saying Congratulations etc and 10/15 minutes later he got another email with the code.