Create `[pmpro_list_addon_users]` shortcode to list addon package (page/post) users who're registered as having purchased the specified list of addon package IDs.

e20r-list-all-addonpage-users.php

<?php
/*
Plugin Name: PMPro Add-on Page/Post user list
Plugin URI: https://eighty20results.com/paid-memberships-pro/do-it-for-me/
Description: List all user(s) who've bought a specific Add-on Post/Page
Version: 1.1
Author: Eighty / 20 Results by Wicked Strong Chicks, LLC <[email protected]>
Author URI: https://eighty20results.com/thomas-sjolshagen/
License:

 * Copyright (C) 2016  Thomas Sjolshagen - Eighty / 20 Results by Wicked Strong Chicks, LLC
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; either version 2
 * of the License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
 */

class e20rAddonPageUsers {

	/**
	 * @var e20rAddonPageUsers
	 */
	static $instance;

	function __construct() {

		add_shortcode('pmpro_list_addon_users', array( $this, 'list_addon_users') );
	}

	/**
	 * Shortcode [pmpro_list_addon_users] to list all users who've been assigned/bought an add-on page/post.
	 *
	 * Requirements: `post_id` - the Post ID(s) for the add-on post/page. Can be a list (comma separated)
	 * Optional: `name_option` - Valid options: `display_name` (default), `first_name` or `last_name`, `user_email`, `user_login`
	 * @param array $attrs
	 *
	 * @return null|string
	 */
	public function list_addon_users( $attrs = array() ) {

		global $wpdb;

		$post_id = null;
		$name_option = '';

		$attrs = shortcode_atts(
			array(
				'post_id' => '', // Can be a comma separated list of Addon Package Page/Post IDs.
				'name_option' => 'display_name'
			),
			$attrs
		);

		if ( !empty( $attrs['name_option'])) {
			$name_option = trim($attrs['name_option']);
		}

		$posts = array_map('trim', explode(',', $attrs['post_id']));
		$post_list = implode(',', $posts ) ;

		// Nothing to show/list.
		if ( empty( $posts ) || empty( $attrs['post_id'] )) {
			return "No Addon Package ID specified";
		}

		// Find the serialized list of users associated with the specified AP posts/pages
		$sql = "SELECT post_id, meta_value 
				FROM {$wpdb->postmeta} 
				WHERE meta_key = '_pmproap_users' 
				AND post_id IN ( {$post_list} )";

		$serialized = $wpdb->get_results( $sql );
		$users = array();

		// Process if we found data
		if (!empty( $serialized ) ) {

			// Create list of users keyed on post IDs
			foreach( $serialized as $record ) {

				$post_id = $record->post_id;
				$user_list = maybe_unserialize( $record->meta_value );

				// Make sure the user list is an array.
				if ( !is_array( $user_list ) ) {
					$user_list = array( $user_list );
				}
				$users[$post_id] = $user_list;
			}
		}

		//Prepare to output the list (per page).
		ob_start();
		foreach( $users as $post => $user_list ) {

		?>
			<div class="e20r-ap-post-list">
				<div class="e20r-ap-post-title">
					<h2><?php echo get_the_title( $post ); ?></h2>
				</div>
				<div class="e20r-ap-post-userlist">
					<ul class="e20r-ap-post-users">
						<?php
						foreach( $user_list as $user_id ) {
							$user = get_userdata( $user_id );
						?><li class="e20r-ap-post-user"><?php echo ( empty($name_option) ?  'Unknown'  : esc_attr( $user->{$name_option} ) ); ?></li>
						<?php
						}
						?>
					</ul>
				</div>
			</div>
		<?php
		}

		$html = ob_get_clean();

		return $html;
	}

	static public function getInstance() {

		if (is_null( self::$instance ) ) {
			self::$instance = new self;
		}

		return self::$instance;
	}
}

add_action('plugins_loaded', 'e20rAddonPageUsers::getInstance' );