Is there a way to have the directory only viewable to registered users? So basically only users that have logged in can see the directory.

easy way to do it would be to make a primary page with a form, then provide them with a user name and pass. then just pass protect the directory. Could do it in html, flash whatever. That would be the easiest way I can think of without knowing allot of php. However not the most efficient way.
Hope that helps.