This is a general warning as I need to check to make sure that variable is set before trying to call implode() on it. It doesn't actually cause any problems though. You can get rid of the error by editing selectusers.php. Line 39 looks like this:
$choice = implode(",", $HTTP_POST_VARS['assignmod']);
Change that, to this:
$choice = @implode(",", $HTTP_POST_VARS['assignmod']);