Reorder default columns on the Members List. In this example, we are moving Username to the first in list.

custom_order_manage_memberslist_columns.php

<?php
/**
 * Reorder default columns on the Members List. In this example, we are moving Username to the first in list.
 *
 * 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 custom_order_manage_memberslist_columns( $columns ) {
	unset( $columns['username'] );
	$new_columns = array();
	$new_columns['username'] = 'Username';
	$columns = array_merge( $new_columns, $columns );
	return $columns;
}
add_filter( 'pmpro_manage_memberslist_columns', 'custom_order_manage_memberslist_columns' );