<?php

  /**
   * Get all users
   */
  $query = db_select('users', 'u')->condition('u.uid', 0, '<>')->fields('u', array('uid'));
  $result = $query->execute();

  /**
   * Loop trough all the users
   */
  foreach ($result as $user) {
    /**
     * Load a user
     */
    $user = user_load($user->uid);

    /**
     * Double check/safe
     */
    if ($user) {
      /**
       * Check if user has value in local group id field
       */
      if (isset($user->field_local_group_id[LANGUAGE_NONE][0]['value']) && !empty($user->field_local_group_id[LANGUAGE_NONE][0]['value'])) {
        /**
         * Get local groups node
         */
        $localGroup = node_load($user->field_local_group_id[LANGUAGE_NONE][0]['value']);

        /**
         * Double check/safe
         */
        if ($localGroup) {
          $values = array(
            'entity_type' => 'user',
            'entity' => $user,
            'state' => OG_STATE_ACTIVE,
            'membership_type' => OG_MEMBERSHIP_TYPE_DEFAULT
          );

          /**
           * Setup a group
           */
          og_group('node', $localGroup->nid, $values);
        }
      }
    }
  }