I've had people search for my email address on facebook, which I have an account for but really don't use. I've also recently gotten an email indicating someone was looking for my email on Twitter. I considered signing up for it but couldn't get the info I needed to feel comfortable with it so never finished the process, even though I was flattered that this person was interested in following what I am up to. Maybe you could do a notification email like Twitter sends when someone searches and you don't yet have an account but with a better explanation of it all so the invitee feels more motivated/comfortable? IE try Twitter's approach but try to improve on their model.