Add a user to a team.

def update(
            self, request, pk=None, parent_lookup_seedteam=None,
            parent_lookup_seedteam__organization=None):
        '''Add a user to a team.'''
        user = get_object_or_404(User, pk=pk)
        team = self.check_team_permissions(
            request, parent_lookup_seedteam,
            parent_lookup_seedteam__organization)
        team.users.add(user)
        return Response(status=status.HTTP_204_NO_CONTENT)