That feature is not built into Outlook. See http://www.outlookcode.com/d/database.htm#tools and http://www.slipstick.com/addins/groupcontacts.htm for possible solutions.

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Sue Mosher, Outlook MVP
Author of
Microsoft Outlook Programming - Jumpstart for
Administrators, Power Users, and Developers
http://www.outlookcode.com/jumpstart.aspx
>I need to link a client database in Access, to Outlook's contacts (so that
> any changes to client information in the Access database, will automatically
> update the corresponding contact in Outlook). If possible, I need the link
> to be uni-directional (from Access to Outlook), so that inadvertent changes
> to Outllook's contacts, will not adversely modify the master client database
> in Access.