2nd pass at hidden category

gistfile1.php

<?php
/*
Plugin Name: Private Events
Plugin URI: http://tri.be
Description: This plugin allows you to have a private event category only visible to viewers who have logged in to your site.
Version: 1.0
Author: Daniel - Coder / Phil - Plugin typer upper
Author URI: http://tri.be
License: A "Slug" license name e.g. GPL2
*/
?>
<?php
add_filter( 'parse_tribe_event_query', 'hide_my_private_category', 999 );

function hide_my_private_category( $query ) {

	/*
	 * First we create the tax_query for excluding
	 * all the events that have the term 'hidden'.
	 * Change that 'hidden' for the slug of the term
	 * you want to hide from the Front End.
	 *
	 */
	$taxquery = array( 'taxonomy' => TribeEvents::TAXONOMY,
	                   'field'    => 'slug',
	                   'terms'    => array( 'private' ),
	                   'operator' => 'NOT IN' );

	/*
	 * The we merge our new tax_query with the
	 * original one, so we don't loose any category
	 * filter we may need to use in the Front End
	 */


	if ( $query->tax_query && property_exists( $query->tax_query, 'queries' ) ) {

		$taxquery = array_merge( $taxquery, $query->tax_query->queries );
	}

	/*
	 * And at the end we add the new resulting tax_query
	 * to the main events query, _before_ said query
	 * so we don't need to traverse the whole results
	 * and delete the events we don't want to show.
	 *
	 * This is the equivalent of using pre_get_posts
	 * to filter things out in the standard WordPress
	 * way
	 *
	 */

	$query->set( 'tax_query', array( $taxquery ) );


	return $query;
}