Create users for every 'fitspirator' node without an entity reference.

create_users_for_nodes

/**
 * Create user for every fitspirator node without any entity reference.
 */
$query = db_select('node', 'n');
$nids = $query
  ->fields('n', array('nid'))
  ->condition('type', 'fitspirator')
  ->execute()
  ->fetchCol();

$nodes = node_load_multiple($nids);
$role = user_role_load_by_name('fitspirator');

foreach ($nodes as $node) {
  if (empty($node->field_fitspirator)) {

    if (!user_load_by_name($node->title)) {

      $edit = array(
        'name' => $node->title,
        'status' => 1,
        'language' => $node->language,
        'roles' => array($role->rid => $role->name),
      );

      $user = user_save(NULL, $edit);

      if ($user) {
        $node->uid = $user->uid;
        $node->field_fitspirator[$node->language][0]['target_id'] = $user->uid;
        node_save($node);
      }

    }
  }
}