<?php //Please do not include the opening PHP tag if you already have one

function tw_ee_custom_Event_Attendees__process_shortcode__order_by( $order_by_array ){
  return array('ATT_fname' => 'ACS');
}
add_filter('FHEE__EES_Espresso_Event_Attendees__process_shortcode__order_by','tw_ee_custom_Event_Attendees__process_shortcode__order_by');