Extending Roles via Plugin

role-extender.php

<?php

/**
 * Plugin Name: Allow Subscribers to Read Private Content
 * Plugin URI: https://brettwidmann.com
 * Description: A plugin that allows subscribers to read private posts and pages
 * Version: 1.0
 * Author: Brett Widmann
 * Author URI: https://brettwidmann.com
 */

register_activation_hook(__FILE__, function()
{
    // Make private posts and pages available to subscribers
    $subRole = get_role( 'subscriber' );
    $subRole->add_cap( 'read_private_posts' );
    $subRole->add_cap( 'read_private_pages' );
});

register_deactivation_hook(__FILE__, function()
{
    // Make private posts and pages unavailable to subscribers
    $subRole = get_role( 'subscriber' );
    $subRole->remove_cap( 'read_private_posts' );
    $subRole->remove_cap( 'read_private_pages' );
});

This gist is for my article at https://brettwidmann.com/extending-role-capabilities-in-wordpress/