$this->db->where('group_id', 'admin');
$this->db->select('id');
$this->db->from('users');

// 쿼리 컴파일
$subQuery = $this->db->_compile_select();

// 액티브 레코드 초기화.
$this->db->_reset_select();

$this->db->select('*');
$this->db->where("user_id IN ($subQuery)", NULL, FALSE);
$this->db->from('boards');

echo $subQuery = $this->db->_compile_select();

//SELECT * FROM (`boards`) WHERE user_id IN (SELECT `id` FROM (`users`) WHERE `group_id` = 'admin')

$this->db->where('user_id', "($subQuery)", FALSE);  => $this->db->where("user_id IN ($subQuery)", NULL, FALSE);