Change group in joomla registration

Change group in joomla registration

I need to change default group when new user registers in Joomla! 2.5.
Right now default group is Registered - number 2 in #__user_usergroup_map
Group_id column.
I tried to change variable here, (e.g $system = '33';) but it didn't worked:
$this->data->groups = array();
$system = $params->get('new_usertype', 2);
$this->data->groups[] = $system;