I received an email saying someone (my mom) added me to a tree. I clicked on the link in the email. I didn't have an account, so I tried to register, only to be told that my email was already in someone's tree and I couldn't be allowed to register. Then I registered independently of the link, clicked on the link again, but still couldn't see the tree my mom had created.
In short, you got the invite/registration functionality wrong...
