You can't. Although each request for a document is usually
logged with the name or address of the remote host, the
actual username is almost never logged as well. This is
mostly because of performance reasons, as it would require
that the server uses the ident protocol to see who is on the
other end. This takes time. And if a cache proxy is doing
the request, you don't get anything sensible.

In Netscape 2.0, it was possible to automatically submit a
form with a mailto as action, using Javascript. This would
send e-mail to the document's owner, with the address the
visitor configured in the From line. Of course, that can be
"mickey.mouse@disney.com". This is fixed in Netscape 2.01.

The most reliable way is to put up a form, asking the
visitor to fill in his e-mail address. If you offer him
something in return, he will most likely do it.