def add_group_grant(self, permission, group_id):
        """
        Convenience method that provides a quick way to add a canonical group
        grant to a key. This method retrieves the current ACL, creates a new
        grant based on the parameters passed in, adds that grant to the ACL and
        then PUT's the new ACL back to GS.

        :type permission: string
        :param permission: The permission being granted. Should be one of:
            READ|FULL_CONTROL
            See http://code.google.com/apis/storage/docs/developer-guide.html#authorization
            for more details on permissions.

        :type group_id: string
        :param group_id: The canonical group id associated with the Google
            Groups account you are granting the permission to.
        """
        acl = self.get_acl()
        acl.add_group_grant(permission, group_id)
        self.set_acl(acl)