Json :
{ "title":"Supervisor",
  "type":"group",
  "og_group_ref": { "und": [{ "target_id": "1921" }]},
  "field_records": { "und": [{ "value": "5" }]}
}

/*
 * Implements hook_services_resources
 */
function your_module_name_services_resources() {
  return array(
    'add_user' => array(
      'create' => array(
        'help' => 'Add users to group',
        'callback' => 'your_module_add_user_to_group',
        'access callback' => TRUE,
        'access arguments append' => TRUE,
        'args' => array(
          array(
            'name' => 'uid',
            'type' => 'id',
            'description' => 'The uid of the user to add in group',
            'source' => array('data' => 'uid'),
            'optional' => FALSE,
          ),
        ),
      ),
    ),
  );
}

/*
 * Call back to add the user
 */
function your_module_add_user_to_group($uid) {
  $user_account = user_load($uid);
  $og_values = array(
     'entity_type' => 'user',
     'entity' => $user_account,
     'state' => OG_STATE_ACTIVE,
   );
   //This is will add the group to the user.
  og_group('node', $your_group_nid, $og_values);
  user_save($user_account);
  return success;
}