Add a user to an organization.

def update(self, request, pk=None, parent_lookup_organization=None):
        '''Add a user to an organization.'''
        user = get_object_or_404(User, pk=pk)
        org = get_object_or_404(
            SeedOrganization, pk=parent_lookup_organization)
        self.check_object_permissions(request, org)
        org.users.add(user)
        return Response(status=status.HTTP_204_NO_CONTENT)