Adds the ACLS that allow a contact to only see contacts and groups associated with the domain (by virtue of them being in the domain group - either directly or via a group connected to the same organisation).
The domain group is configured through the administration menu within CiviCRM under System Settings/Multisite Settings.
Note that previous versions of the multisite extension made heavy use of Group Nesting. This is no longer recommended for performance reasons. Avoid group nesting where possible.