Example of how to hook into the finalize registration step and log details to the error log.

tw_ee_log_process_reg_step__completed.php

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

//Debug function to easily write various vars to the log.
if ( ! function_exists('tw_ee_write_log')) {
   function tw_ee_write_log( $log )  {
      if ( is_array( $log ) || is_object( $log ) ) {
         error_log( print_r( $log, true ) );
      } else {
         error_log( $log );
      }
   }
}

add_action(
    'AHEE__EE_SPCO_Reg_Step_Finalize_Registration__process_reg_step__completed',
    'tw_ee_log_process_reg_step__completed',
    10,
    2
);
function tw_ee_log_process_reg_step__completed($checkout, $txn_update_params) {
    if (! $checkout->transaction_has_primary_registrant()) {
        tw_ee_write_log(
            array(
                'Checkout' => $checkout,
                'txn_update_params' => $txn_update_params
            )
        );
    }
}