try another thing

booking.emails.inc

@@ -36,7 +36,6 @@ function booking_manual_email()
 		'#options' => $email_options_array,
 	);
 	
-	/*
 	$form['select-type'] = array(
 			'#type' => 'select',
 			'#title' => t('Pre-Select People'),
@@ -48,7 +47,6 @@ function booking_manual_email()
 				'event' => 'change',
 			),
 	);
-	*/
 	
 	$header = array(
 		'booking_nid' =>  array('data' => t('Id'), 'field' => 'nid', 'sort' => 'asc'),
@@ -257,7 +255,7 @@ function _booking_manual_email_selecttype_dropdown_callback($form, &$form_state)
 		'#multiple' => TRUE,
 		'#header' => $header,
 		'#options' => $options,
-		'#default_value' => array("1814" => TRUE),
+		'#value' => array("1814" => TRUE),
 		'#empty' => t('No attendees found.'),
 		'#attributes' => array('id' => 'sort-table'),
     '#prefix' => '<div id="manual-email-attendees-items">',