public function addGuest($id,$guest) {
        if(is_string($guest)) {
            $params = array('email'=>$guest);
        } elseif(is_numeric($guest)) {
            $params = array('user_id'=>$guest);
        }else  {
            throw new \Exception('Invalid parameter');
        }
         
        return $this->post('/projects/'.$id.'/add_guest.json', $params);
        
    }