Everyone participating in NaNoWriMo gets a little mini-email program built into their user account. It's called "NaNoMail," and it allows you to send private messages to other participants without having to know their actual email address. You'll see a "Check and Send NaNoMail" link on your Author Profile page.
NaNoMail is turned on by default. To make it so participants can't send you NaNoMail, just sign in, then head to My NaNoWriMo-->Edit User Settings and uncheck the box marked "Allow Private Messages."