This function sets the Registration CSV report to only export primary registrants.

example.php

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


function tw_ee_only_export_primary_registrants($reg_csv_array, $reg_row) {
    //Only export the primary registrants
    if( $reg_row['Registration.REG_count'] !== '1' ) {
         return array();
    }

    //Include the REG_Group_Size column so you can see how many tickets purchased.
    $reg_csv_array['Group Size'] = $reg_row['Registration.REG_group_size'];
    
    return $reg_csv_array;
}
add_filter( 'FHEE__EventEspressoBatchRequest__JobHandlers__RegistrationsReport__reg_csv_array', 'tw_ee_only_export_primary_registrants', 100, 2 );