code: function(uid, sid, groups) {
    if(!sid) {
      logger.warn('ignore uid %j for sid not specified.', uid);
      return false;
    }
  
    var group = groups[sid];
    if(!group) {
      group = [];
      groups[sid] = group;
    }
  
    group.push(uid);
    return true;
  }
docs: add uid and sid into group. ignore any uid that uid not specified.
  
  @param uid user id
  @param sid server id
  @param groups {Object} grouped uids, , key: sid, value: [uid]