code: function (key, devices) {
      if (_.isUndefined(devices)) { // Just create group without any device.
        refs.userGroups.child(key).set(false, function (err) {
          if (!err) debug('Empty group created: ', key);
        });
      } else {
        refs.userGroups.child(key).set(devices);
      }
    }
docs: /*
  Group Support