I suspect that what you're trying to achieve is actually best accomplished by using multiple Address Lists, which are different from GALs. An organization can have many Address Lists, and these are simply configured by creation of the AL in your Exchange Management Console. You can then configure an LDAP query that captures all the relevant users for the AL.
For example, you could create an AL for any user with "Department" set to "Corporate Services," then any user created in Active Directory provisioned with a mailbox and the "Department" attribute appropriately populated will show up when the Outlook user selects the appropriate AL. The Outlook client triggers the LDAP query whenever the user opens the AL, therefore, updates are immediate.
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