Another thing we might consider is making it a little backwards -- when someone submits their address book, they already have an account. We can make it so that a user can select a privacy setting of "when someone submits my email, notify me, but don't tell them".

That was going to be my suggestion as well. If my aunt joins and searches for me with the email address my fucking brother outed me with, I don't want her to automatically be able to see that that email address is attached to jumpuphigh.