[Andreas]

Yes. In memberlist.php, just some lines below the place where you added the code there is

PHP Code:

default:
        $sqlsort = 'user.username';
        $sortfield = 'username'; 


Replace that with

PHP Code:

default:
        $sqlsort = 'userfield.field5';
        $sortfield = 'companyname'; 


(Untested as I was too lazy to test, but theoretically it should work )