Ronx & Kevin,
I will rephrase the question.
If someone logs onto my website, is there a way to capure their email
address so that I can send them additional information? Or do I need to have
them sign a guest book?
Your visitors will, at some stage (most likely the registration
stage), have to fill in a form that asks for their email address. You
will store that address along with their logon userid ands password.
The form could be a registration form, guestbook, or application to go
on a mailing list.
Note that visitors from Europe will need to be able to "opt in" to
receive emails from your site, in order to comply with EEC laws.