Show list of users that purchased a post

shortcode-example.php

<?php
/**
 * Use shortcode [posts_users_example post_id='12']
 * Tweak code to your liking, this is a starter example.
 * Add code via this method - https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
 function my_shortcode_example( $atts ) {

	if ( empty( $atts['post_id'] ) ) {
		return 'no post ID supplied';
	}

	$post_users = get_post_meta( $atts['post_id'], '_pmproap_users', true );

	// Loop through posts and get users belonging to that post.
	if ( is_array( $post_users ) ) {
		$list = '<ul>';
		foreach( $post_users as $key => $user_id ) {
			$user = get_user_by( 'ID', $user_id );

			$list .= '<li>' . $user->user_login . '</li>';
		}
		$list .= '</ul>';
	}

	return $list;
}
add_shortcode( 'posts_users_example', 'my_shortcode_example' );