pmpromd-id-as-user-identifier.php

 <?php
/**
 * Set the user ID as the identifier for the Member Directory.
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
function my_pmpromd_id_as_user_identifier( $identifier ) {

	// Set the identifier to the user's User ID
	$identifier = 'id'; // accepts "slug" (default) or "id"

	return $identifier;
}
add_filter( 'pmpromd_user_identifier', 'my_pmpromd_id_as_user_identifier' );