Polling email on the server is awfully wasteful. I like the "request an invite" pattern. Get an email address and nothing more, send the "invite", clear the for

Polling email on the server is awfully wasteful. I like the "request an invite" pattern. Get an email address and nothing more, send the "invite", clear the form, tell them if they don't get an email in 5 minutes, check spam, still nothing, reenter the email. Don't associate the link with any particular session, just the entered address so there is no risk of leaking any private info, if they type in an address that actually belongs to someone else the only options the other person would have is ignore the email or sign up themselves. When they finally get their email right they'll get a link to a place where they can fill in the rest of the info and you can be assured they gave you the email address they wanted you to have.