You want to limit the People Picker control on the site-collection level to find users in a specific organization unit (OU) in a trusted domain. However, you cannot use the stsadm -o setsiteuseraccountdirectorypath command to work with OUs from a trusted domain. You can use the stsadm -o setsiteuseraccountdirectorypath command only when the OU is from the same domain where the Microsoft Office SharePoint Sever (MOSS) 2007 farm is installed.
To resolve this issue, install the hotfix package that is described in the following Microsoft Knowledge Base article:
961750 Description of the Windows SharePoint Services 3.0 hotfix package (Sts.msp): February 24, 2009
After you install this hotfix, follow these steps to enable LDAP referral chasing for trusted domains. This makes sure that the stsadm -o setsiteuseraccountdirectorypath command to work with OUs from a trusted domain:
Visit the following Microsoft Web site to download Windows PowerShell: