aMember (site.php): How to add new columns to grids in admin interface

site.php

<?php

Am_Di::getInstance()->hook->add('gridProductInitGrid', function(Am_Event_Grid $e) {

    $e->getGrid()->addField('comment', ___('Comment'));

    $e->getGrid()->addField('start_date', ___('Start Date Calculation'))
        ->setGetFunction(function($record, $grid, $fieldname, $field){
            $r = explode(',', $record->{$fieldname});
            foreach ($r as $k => $v) {
                if ($v == 'group' && $record->renewal_group) {
                    $r[$k] = sprintf('group (%s)', $record->renewal_group);
                }
                if ($v == 'fixed') {
                    $r[$k] = sprintf('fixed (%s)', $record->start_date_fixed);
                }
            }
            return implode(', ', $r);
        });

});