example2.php

$_product = Mage::getModel('catalog/product')->setStoreId($webId)->loadByAttribute('sku', $arrprice['sku']);

$productEntity = Mage::getModel('catalog/product');

$productEntity->setStoreId(Mage_Core_Model_App::ADMIN_STORE_ID)
    	      ->load($id);
$groupPrices = array(
	     array('website_id' => $webId, 'cust_group' => $cutGrpId, 'all_groups' => false,'price' => $arrprice['someprice'])
	);
  
  try{
      	 $productEntity->setData('group_price', $groupPrices)		      
         ->save();
	 
	 }catch	(Exception $e)
	    {
	    Mage::log(print_r($e,true), NULL, 'my_exception.log',true); 
	    }