Er, I think you may have been accidentally trying to make a new account rather than sending your friend the link. Our invite system isn't one where you put in an email and Iwaku sends them a link to the sign up page. The link is for other people to click on, not you.
What you have to do is go get that link FieryCold directed you to and send that link to your friend. Copy and paste it into an email, text message, whatever works. Then they have to click your invite link, which takes them to the page to make an account.