<?php $event_options = array(
	'conditions' => array(
		'Event.execute_date < NOW()',
		'Event.is_triggered' => 0
	),
	'fields' => 'Event.campaign_id',
	'recursive' => -1
);
$campaign_ids_sql = $this->Event->find('sql', $event_options);

// SELECT `Event`.`campaign_id` FROM `events` AS `Event` 
// WHERE `Event`.`execute_date` < NOW() 
// AND `Event`.`is_triggered` = 0

$user_options = array(
	'conditions' => array(
		'Group.campaign_id IN ('.$campaign_ids_sql.')'
	),
	'contain' => 'Group',
	'fields' => 'User.id'
);
$user_ids = $this->User->find('all', $user_options);

// SELECT `User`.`id` FROM `users` AS `User` 
// LEFT JOIN `groups` AS `Group` ON (`User`.`group_id` = `Group`.`id`) 
// WHERE `Group`.`campaign_id` IN (
//   SELECT `Event`.`campaign_id` FROM `events` AS Event 
//   WHERE `Event`.`execute_date` < NOW() AND `Event`.`is_triggered` = 0 
// )
?>