Regarding rostering people who can see this file, this is also possible.
Whenever a person gets access to lan they have a user name (either logging
through windows envoirnement, Novell, citrix etc). You can write a code to check
the user name and have a list of people who should have access on a seperate
sheet. This way if the user name (log in name) matches what is written on the
tab, then you would be able to restrict people to see the file.

You can also give access level so they can edit the file or not.