@Override
    public boolean addIfAbsent(T e) {
        boolean added = super.addIfAbsent(e);
        if (added) {
            syncGroups(e, ADD);
        }
        return added;
    }
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