the safe way of doing this is when you send the invite it takes you to a page that says
"Great succes beta key sent
....
beta key: "aafio368asjs"
Make sure you copy the beta key displayed and email it to them yourself, i invited a friend and he never got the email so i just sent him the beta jey and he manually enterred it and it worked
I have the same problem as well. I have used my 3 invites, but none of the mail with the keys has arrived. I copyed one of the keys and was able to create a new account, but there was no invites on the new account