You should searxh Administrator's manual for: Detecting identity from a synchronized directory group server.

1. In DLP console you should go first to System > Settings > Group Directories and there press Create New Connectiom button. Fill out the fields accordingly.
2. Then use Manage > User Groups and add groups based on your group directory connection from previous step.
3. Then go to Manage > Policies and choose (or create new one). And then use Group tab.