<?php
$installer = $this;
$installer->startSetup();
$installer->getConnection()->addColumn(
    $this->getTable('newsletter/subscriber'),//table name

    'groupid',      //column name
    'varchar(100) NOT NULL'  //datatype definition
    );

$installer->endSetup();

?>

<modules>
            <Mage_Newsletter>
                     <version>1.6.0.0</version>
            </Mage_Newsletter>
    </modules>