public PersistableMap<String,String> getProperties() {
        synchronized (this) {
            if (properties == null) {
                properties = provider.loadProperties(this);
            }
        }
        // Return a wrapper that will intercept add and remove commands.
        return properties;
    }
Returns all extended properties of the group. Groups have an arbitrary
number of extended properties. The returned collection can be modified
to add new properties or remove existing ones.

@return the extended properties.
xmppserver/src/main/java/org/jivesoftware/openfire/group/Group.java