Event Espresso 4, Infusionsoft. Add a custom field value for "Primary Attendee" to guest contact records.

eea_is_track_guests_to_primary.php

<?php
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file

function ee_is_track_attendee_back_to_purchaser( $is_contact_data, $ee_attendee ) {
    if( $ee_attendee instanceof EE_Attendee ) {
      $checkout = EE_Registry::instance()->SSN->checkout();
      if ( $checkout instanceof EE_Checkout ) {
        $transaction = $checkout->transaction;
        if ( $transaction instanceof EE_Transaction ) {
          $primary_reg = $transaction->primary_registration();
          if ($primary_reg instanceof EE_Registration) {
            $attendee = $primary_reg->attendee();
            if ($attendee instanceof EE_Attendee) {
              if ($attendee->id() != $ee_attendee->id()) {
                $is_contact_data[ '_PrimaryContact' ] = $primary_reg->attendee()->full_name();
              }
            }
          }
        }
      }
    } else {
        EE_Error::add_error(sprintf( 
          __( 
          'ee_is_track_attendee_back_to_purchaser was not called with an EE_Attendee but a %s', 
          'event_espresso' 
        ), gettype( $ee_attendee )), __FILE__, __FUNCTION__, __LINE__ );
    }
    return $is_contact_data;
}
add_filter(
  'FHEE__EED_Infusionsoft__save_infusionsoft_attendee__extra_attendee_data',
  'ee_is_track_attendee_back_to_purchaser',
  10,
  2
);