Add Company column in customer Grid magento

Go to /app/code/core/Mage/Adminhtml/Block/Customer/Grid.php

in _prepareCollection() add following file
->joinAttribute('billing_company', 'customer_address/company', 'default_billing', null, 'left')

and in _prepareColumns() add following code,

$this->addColumn('company', array(
            'header'    => Mage::helper('customer')->__('Company'),
            'index'     => 'billing_company'
        ));


Note: its the best practise that dont make any changes in your core file. overwrite your php file in local and then make any changes.