doctrine2/models/Group.php

<?php

require_once('Entity.php');

// grouptable doesn't extend Doctrine_Table -> Doctrine_Connection
// won't initialize grouptable when Doctrine_Connection->getTable('Group') is called
class GroupTable { }

class Group extends Entity
{
    public static function initMetadata($class)
    {
        $class->hasMany('User', array(
            'local' => 'group_id',
            'foreign' => 'user_id',
            'refClass' => 'Groupuser'
        ));
    }
}