When customer click on Sign up to get notified when this product is back in stock at that time customer id save in product_alert_stock table

you can get customer id using this model

$model = Mage::getModel('productalert/stock');

Here is code to get customer email address

  $collection = Mage::getModel('productalert/stock')->getCollection();
  $collection->getSelect()->join( array('customer'=> customer_entity), 'customer.entity_id = main_table.customer_id', array('customer.email'));

When a customer clicks on Sign up to get notified when this product is back in stock, the customer id is saved in the product_alert_stock table.

You can get the customer id using this model:

$model = Mage::getModel('productalert/stock');

Here is code to get a customer's email address:

  $collection = Mage::getModel('productalert/stock')->getCollection();
  $collection->getSelect()->join( array('customer'=> customer_entity), 'customer.entity_id = main_table.customer_id', array('customer.email'));