More precision in your answers would be helpful. By "public calendar," do you mean calendars in the Public Folders hierarchy on an Exchange server? If so, the folder owner can change permissions in Outlook by bringing up the folder's Properties dialog and switching to the Permissions tab. See http://www.slipstick.com/exs/permissions.htm for other tools.
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Sue Mosher, Outlook MVP
Author of Configuring Microsoft Outlook 2003
and Microsoft Outlook Programming - Jumpstart for
Administrators, Power Users, and Developers