There is no messaging system; visit the user's profile (click on their name) and cross your fingers they have some kind of identifying information- an email address, a web page, etc.
but I see no e-mail. When I do the
same click on my Hacker News user name,
then, yes, I see a line for my e-mail
but the line (a text box) is empty
because I haven't given my e-mail to
HN.