Outlook PST files

Yesterday, a client had some hard questions for me. One I really couldn’t answer.
He has about 20 PC’s (networked with NT4) all using Outlook.
All these people use shared mailboxes. Because the number of read messages became to high, they needed to archive them into PST files and put them on a network drive. One user can open the file, but the second one will get a sharing violation message. Only if the first closes down Outlook the file becomes readable again. But now comes the tricky part.How can you, in an easy way, share this kind of files for all the users. So that it’s always there in Outlook AND accesible for everyone at the same time.

Pete