@Override
    public void add(int index, T element) {
        super.add(index, element);
        syncGroups(element, ADD);
    }
This method is called from several of the mutators to keep
the group set in sync with the full list. 

@param item The item to be added or removed if it is in the group set
@param addOrRemove True to add, false to remove
@return true if the given item is a group
xmppserver/src/main/java/org/jivesoftware/openfire/group/ConcurrentGroupList.java