public function addInterestGroup($name, $group_id = NULL)
    {

        $payload = array(
            'id' => $this->listId,
            'group_name' => $name,
            'grouping_id' => (NULL === $group_id) ? $this->grouping_id : $group_id,
        );

        $apiCall = 'lists/interest-group-add';
        $data = $this->requestMonkey($apiCall, $payload);
        $data = json_decode($data, true);
        if (isset($data['error']))
            throw new MailchimpAPIException($data);
        else
            return true;
    }