The answer depends on which version of Outlook and Exchange you are utilizing.
For users who connect to Microsoft Exchange server, maintaining a contacts folder in the Public Folders hierarchy is one way to benefit from a group address book that individual users can contribute to. With some extra effort (see below), the Outlook Address Book can also display contacts from another user's mailbox, making it easy for an assistant to use the boss' contacts to address messages.
The process of adding another user's Contacts folder to your own address book display is somewhat involved:
1. Create a new Outlook profile that connects directly with the other user's mailbox, not your own, then start Outlook with this profile. If you receive a prompt that Outlook cannot open the folders, you may need to ask the Exchange administrator to grant your Windows user account full access to the other user's mailbox temporarily so that you can complete these steps.
2. Right-click the other user's Contacts folder, choose Properties, then switch to the Outlook Address Book tab. Make sure that the Show this folder as an e-mail Address Book box is checked. Under Name of the address book, change the displayname from Contacts (the default) to a name that will distinguish this address list, such as Joe's Contacts if you're in Joe's mailbox.
3. Close Outlook.
4. In Control Panel | Mail, bring up the properties for the profile that you created in Step 1. In the Microsoft Exchange Server service, change the main mailbox from the other user's to your own. On the Advanced tab of the Exchange Server service, add the other user's mailbox as a secondary mailbox.
5. Restart Outlook, using the same profile as in Steps 1 and 4.