how to restrict users to search in their own Organizational Unit



We are currently implementing MOSS2007.
However if a user is trying to add users and he looks up users in
Active Directory, then he can see all users in Active Directory.
How can you limit a user within an Organizational Unit so he can only
search for users within his OU?

We have:

DC=domain, DC=com
OU=companyA, DC=domain, DC=com
OU=companyB, DC=domain, DC=com
OU=companyC, DC=domain, DC=com

Ideal scenario would be that users from OU=companyA can only search
within their OU=companyA, and cannot see users in the other OU's.

.