public void addUserIdentitiesProvider(UserIdentitiesProvider provider) {
        if ( provider == null )
        {
            throw new NullPointerException( "Argument 'provider' cannot be null." );
        }
        registeredUserIdentityProviders.add( provider );
    }
Adds the "discoverable" user identities provided by the provider whenever a disco for info is made against users
of the server.

@param provider The provider of user identities.
xmppserver/src/main/java/org/jivesoftware/openfire/disco/IQDiscoInfoHandler.java