Adding columns to customer grid using observer or overriding the customer grid

The Observers way:

Declare 2 observers in your config.xml file: one to add your column to the grid block and the other one to load data from the corresponding attribute:

<adminhtml>
    <events>
        <core_block_abstract_to_html_before>
            <observers>
                <{observer_name}>
                    <type>singleton</type>
                    <class>{namespace}_{module}/observer</class>
                    <method>beforeBlockToHtml</method>
                </{observer_name}>
            </observers>
        </core_block_abstract_to_html_before>
        <eav_collection_abstract_load_before>
            <observers>
                <{observer_name}>
                    <class>{namespace}_{module}/observer</class>
                    <method>beforeCollectionLoad</method>
                </{observer_name}>
            </observers>
        </eav_collection_abstract_load_before>
    </events>
</adminhtml>

Create an Observer class with appropriate methods:

class {Namespace}_{Module}_Model_Observer
{
    public function beforeBlockToHtml(Varien_Event_Observer $observer)
    {
        $grid = $observer->getBlock();

        /**
         * Mage_Adminhtml_Block_Customer_Grid
         */
        if ($grid instanceof Mage_Adminhtml_Block_Customer_Grid) {
            $grid->addColumnAfter(
                '{column_code}',
                array(
                    'header' => Mage::helper('{Module}_customer')->__('{{column_name}}'),
                    'index'  => '{column_code}'
                ),
                'entity_id'
            );
        }
    }

    public function beforeCollectionLoad(Varien_Event_Observer $observer)
    {
        $collection = $observer->getCollection();
        if (!isset($collection)) {
            return;
        }

        /**
         * Mage_Customer_Model_Resource_Customer_Collection
         */
        if ($collection instanceof Mage_Customer_Model_Resource_Customer_Collection) {
            /* @var $collection Mage_Customer_Model_Resource_Customer_Collection */
            $collection->addAttributeToSelect('{attribute_code}');
        }
    }
}

To add a column to the customer grid, you need to override 2 things in the block Mage_Adminhtml_Block_Customer_Grid.

• _prepareCollection - to add your attribute in the collection
• _prepareColumns - to add the column in your grid.

For this you should create a new extension. Let's call it Easylife_Customer. For this you will need the following files:
app/etc/module/Easylife_Customer.xml - the declaration file

<?xml version="1.0"?>
<config>
    <modules>
        <Easylife_Customer>
            <active>true</active>
            <codePool>local</codePool>
            <depends>
                <Mage_Customer /><!-- your module should depend on Mage_Customer -->
                <Mage_Adminhtml /><!-- your module should depend on Mage_Adminhtml also -->
            </depends>
        </Easylife_Customer>
    </modules>
</config>

app/code/local/Easylife/Customer/etc/config.xml - the configuration file

<?xml version="1.0"?>
<config>
    <modules>
        <Easylife_Customer>
            <version>0.0.1</version>
        </Easylife_Customer>
    </modules>
    <global>
        <blocks>
            <adminhtml>
                <rewrite>
                    <customer_grid>Easylife_Customer_Block_Adminhtml_Customer_Grid</customer_grid><!-- rewrite the customer grid -->
                </rewrite>
            </adminhtml>
        </blocks>
    </global>
</config>

app/code/local/Easylife/Customer/Block/Adminhtml/Customer/Grid.php - your own version of the customer grid. Read my comments in the code:

<?php
class Easylife_Customer_Block_Adminhtml_Customer_Grid extends Mage_Adminhtml_Block_Customer_Grid{
    /**
     * override the _prepareCollection to add an other attribute to the grid
     * @return $this
     */
    protected function _prepareCollection(){
        $collection = Mage::getResourceModel('customer/customer_collection')
            ->addNameToSelect()
            ->addAttributeToSelect('email')
            ->addAttributeToSelect('created_at')
            ->addAttributeToSelect('group_id')
            //if the attribute belongs to the customer, use the line below
            ->addAttributeToSelect('mobile')
            //if the attribute belongs to the customer address, comment the line above and use the one below
            //->joinAttribute('mobile', 'customer_address/mobile', 'default_billing', null, 'left')
            ->joinAttribute('billing_postcode', 'customer_address/postcode', 'default_billing', null, 'left')
            ->joinAttribute('billing_city', 'customer_address/city', 'default_billing', null, 'left')
            ->joinAttribute('billing_telephone', 'customer_address/telephone', 'default_billing', null, 'left')
            ->joinAttribute('billing_region', 'customer_address/region', 'default_billing', null, 'left')
            ->joinAttribute('billing_country_id', 'customer_address/country_id', 'default_billing', null, 'left');

        $this->setCollection($collection);
        //code from Mage_Adminhtml_Block_Widget_Grid::_prepareCollection()
        //since calling parent::_prepareCollection will render the code above useless
        //and you cannot call in php parent::parent::_prepareCollection()
        if ($this->getCollection()) {

            $this->_preparePage();

            $columnId = $this->getParam($this->getVarNameSort(), $this->_defaultSort);
            $dir      = $this->getParam($this->getVarNameDir(), $this->_defaultDir);
            $filter   = $this->getParam($this->getVarNameFilter(), null);

            if (is_null($filter)) {
                $filter = $this->_defaultFilter;
            }

            if (is_string($filter)) {
                $data = $this->helper('adminhtml')->prepareFilterString($filter);
                $this->_setFilterValues($data);
            }
            else if ($filter && is_array($filter)) {
                $this->_setFilterValues($filter);
            }
            else if(0 !== sizeof($this->_defaultFilter)) {
                $this->_setFilterValues($this->_defaultFilter);
            }

            if (isset($this->_columns[$columnId]) && $this->_columns[$columnId]->getIndex()) {
                $dir = (strtolower($dir)=='desc') ? 'desc' : 'asc';
                $this->_columns[$columnId]->setDir($dir);
                $this->_setCollectionOrder($this->_columns[$columnId]);
            }

            if (!$this->_isExport) {
                $this->getCollection()->load();
                $this->_afterLoadCollection();
            }
        }

        return $this;
    }

    /**
     * override the _prepareColumns method to add a new column after the 'email' column
     * if you want the new column on a different position just change the 3rd parameter
     * of the addColumnAfter method to the id of your desired column
     */
    protected function _prepareColumns(){
        $this->addColumnAfter('mobile', array(
            'header'    => Mage::helper('customer')->__('Mobile'),
            'index'     => 'mobile'
        ),'email');
        return parent::_prepareColumns();
    }
}

Clear the cache and you should be ready.

I reply to Alex comment :

To export in CSV too use

core_block_abstract_prepare_layout_before

instead of

core_block_abstract_to_html_before