As far as I know, the outlook address book is combined in the pst file, so it cant be opened by more than one person at the same time (we tried this at work for the address book).
With Outlook Express, it uses the "Address Book" which comes with Windows, and therefore uses the "*.wab" file which to the best of my knowledge CAN be opened by more than one person. Might be worth putting some contacts into the Window's Address Book (Start, All Programs, Accessories, Address Book) and then export the file out to a server location. I think you could change something in the registry to tell Windows that the *.wab file is at a different location than the normal one.