Adding Capabilities to Roles in WordPress via Hook

add-cap-functions.php

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

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