Search member directory for custom user meta passed in via query parameters.

my_pmpro_member_directory_sql.php

<?php

 /*
 * Search member directory for custom user meta passed in via query parameters.
 *
 * (e.g. www.example.com/directory?company=abc+company)
 */
function my_pmpro_member_directory_sql( $sqlQuery ) {

	// usermeta key to search.
	$meta_key = 'company';

	// Only continue if a value was passed.
	if ( empty( $_REQUEST[ $meta_key ] ) ) {
		return $sqlQuery;
	}

	$meta_value = sanitize_text_field( $_REQUEST[ $meta_key ] );

	global $wpdb;

	$sqlQuery = str_replace( 'WHERE', "LEFT JOIN $wpdb->usermeta umc ON umc.user_id = u.ID AND umc.meta_key = '{$meta_key}' WHERE", $sqlQuery );
	$sqlQuery = str_replace( 'ORDER BY', "AND umc.meta_key = '{$meta_key}' AND umc.meta_value = '{$meta_value}' ORDER BY", $sqlQuery );

	return $sqlQuery;
}
add_filter( 'pmpro_member_directory_sql', 'my_pmpro_member_directory_sql' );