public function create($account, $groupOwner, $groupSlug, $privilege)
    {
        if (!in_array($privilege, array('admin', 'collaborator'))) {
            throw new \InvalidArgumentException("Invalid privilege provided.");
        }

        return $this->requestPost(
            sprintf('users/%s/privileges/%s/%s', $account, $groupOwner, $groupSlug),
            array('privileges' => $privilege)
        );
    }