If you need add to newsletter subscribers grid the customer group (NOT LOGGEED, General, Wholesale, Retail etc) and want use this comlumn for sort, filter I have one solution to how to do it. The main point why I rewrote the model, block and did not use the Observer is used for unregistered subscribers NOT_LOGGED group and the ability to filter and sort by it, now describe in more detail.

As we know we can get from main tabel with JOIN LEFT from customer_entity table customer group and values wiil be for standart General = 1, Wholesale = 2 and Retail = 3 but for unregistered user it will NULL and we can’t user sort and filter by this column. For solve this I rewrote method showCustomerInfo() of Mage_Newsletter_Model_Resource_Subscriber_Collection class to add:

  ->joinLeft(
      array('ce'=>new Zend_Db_Expr('( SELECT COALESCE(customer_id, 0) AS entity_id, COALESCE(ce.group_id, 0) AS group_id FROM newsletter_subscriber AS ns LEFT JOIN customer_entity AS ce ON ns.customer_id = ce.entity_id )')),
      'ce.entity_id=main_table.customer_id',
      array('customer_group_id'=>'group_id')
  )

Pay attention to this point

   new Zend_Db_Expr('( SELECT COALESCE(customer_id, 0) AS entity_id, COALESCE(ce.group_id, 0) AS group_id FROM newsletter_subscriber AS ns LEFT JOIN customer_entity AS ce ON ns.customer_id = ce.entity_id )

I’m replace NULL value for customer group id and customer id to 0 for use it in my custom filter method.

Full method code:

    public function showCustomerInfo()
    {
        $adapter = $this->getConnection();
        $customer = Mage::getModel('customer/customer');
        $firstname  = $customer->getAttribute('firstname');
        $lastname   = $customer->getAttribute('lastname');

        $this->getSelect()
            ->joinLeft(
                array('customer_lastname_table'=>$lastname->getBackend()->getTable()),
                $adapter->quoteInto('customer_lastname_table.entity_id=main_table.customer_id
                 AND customer_lastname_table.attribute_id = ?', (int)$lastname->getAttributeId()),
                array('customer_lastname'=>'value')
            )
            ->joinLeft(
                array('customer_firstname_table'=>$firstname->getBackend()->getTable()),
                $adapter->quoteInto('customer_firstname_table.entity_id=main_table.customer_id
                 AND customer_firstname_table.attribute_id = ?', (int)$firstname->getAttributeId()),
                array('customer_firstname'=>'value')
            )
            ->joinLeft(
                array('ce'=>new Zend_Db_Expr('( SELECT COALESCE(customer_id, 0) AS entity_id, COALESCE(ce.group_id, 0) AS group_id FROM newsletter_subscriber AS ns LEFT JOIN customer_entity AS ce ON ns.customer_id = ce.entity_id )')),
                'ce.entity_id=main_table.customer_id',
                array('customer_group_id'=>'group_id')
            )
            ->group('subscriber_id');

        return $this;
    }

Than I rewrote method _prepareColumns() of class Mage_Adminhtml_Block_Newsletter_Subscriber_Grid for add new colunm:

  $groups = Mage::getResourceModel('customer/group_collection')->load()->toOptionHash();
    $this->addColumn('customer_group_id', array(
        'header'    => Mage::helper('newsletter')->__('Group'),
        'index'     => 'customer_group_id',
        'type'      => 'options',
        'default'   => 0,
        'options'   => $groups,
        'filter_condition_callback' => array($this, '_customerGroupFilter')
  ));

and add custom filter method namely _customerGroupFilter()

  protected function _customerGroupFilter($collection, $column) 
  {
    $value = $column->getFilter()->getValue();
       
    $this->getCollection()->getSelect()->where("ce.group_id =?", $value);
       
   return ;
  }

And thats all.

Read about the SQL COALESCE for more info.