|To add a forest|
In the Group Policy Management Console (GPMC) console tree, right-click Group Policy Management, and then click Add Forest.
In the Add Forest dialog box, type the DNS or NetBIOS name of any domain in the forest, and then click OK.
You can specify either the DNS name or the NetBIOS name of any domain in the forest. If you specify a NetBIOS name, you must confirm that the NetBIOS name corresponds to the DNS name of the domain.
Adding a forest to GPMC does not create a new forest in Active Directory.
Group Policy objects cannot be linked outside a forest.
GPMC supports management of multiple forests from within the console, when there is trust between the target forest and the forest of your user object.
You can use the Add Forest function to add externally trusted domains, even if you do not have forest trust with the entire forest. However, you must have trust between the domain you want to add and the domain of your user object.