Shared contact lists

I have a manager with two secretaries who wish to share her contact list, ideally having real-time access to it as she updates it. So I would like to know the best way of all three of them being able to update and constantly share a common contact list (they use either Outlook 97 or 2000 on an Exchange 5.5. NT server).

Also, on that basis, does anyone know of a solution (or an add-in) that could provide similar firm-wide access to a communal address book?

Many thanks.