Sort the Member Directory by "companyname".

my_pmpro_member_directory_sql.php

<?php
/*
 * Sort the Member Directory by "companyname".
 * Update the shortcode to include [pmpro_member_directory order_by="umc.meta_value"]
 *
 */
function my_pmpro_member_directory_sql( $sqlQuery ) {
	global $wpdb;
	$sqlQuery = str_replace("WHERE mu.status", "LEFT JOIN $wpdb->usermeta umc ON umc.user_id = u.ID AND umc.meta_key = 'companyname' WHERE mu.status", $sqlQuery);
	return $sqlQuery;
}
add_filter('pmpro_member_directory_sql', 'my_pmpro_member_directory_sql');