In OUtlook 2007, use Namespace.GetSelectNamesDialog. In earlier versions, consider using Outlook Redemption.
FYI, this is a newsgroup for Outlook custom forms issues. There is a newsgroup specifically for general Outlook programming issues "down the hall" at microsoft.public.outlook.program_vba or, via web interface, at
http://www.microsoft.com/office/community/en-us/default.mspx?dg=microsoft.public.outlook.program_vba
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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