Registrants for an event via API (https://github.com/dpi/rng)

registrants.php

$event = entity_load( your event entity here )
$event_manager = \Drupal::service('rng.event_manager');
$event_meta = $event_manager->getMeta($event);
foreach ($event_meta->getRegistrations() as $registration) {
  foreach ($registration->getRegistrants() as $registrant) {
    $registrant->label();
    $registrant->id();
  }
}