As alternate e-mail address is possible to use e-mail address actually awaiting for invitation (I try it and succeed with address registered about one hour ago).
Maybe is it feature, maybe no :-)
Only one account is allowed per email address. If you have requested an invite with the same email address, once you get an invitation you can signup using a different email address. There is no restriction on the email used while signing up from invitation link.
Okej, I just try what happened when I fill in used e-mail address ;-) Have a nice day.
honzahommer If you try to signup from the same email, it would throw you an error. 😄