remove original action and create a new one.

Override GBS subscription

function group_buying_subscription_new()
{
	
	if( isset( $_POST['set-group-buying-subscription'] ) && !isset( $_POST['email_address'] ) ) {
		setcookie( 'your-selected-location', $_POST['deal_location'], time( ) + 24 * 60 * 60 * 30, '/' );
		wp_redirect( site_url() );
		exit();
	}
	elseif( isset( $_POST['set-group-buying-subscription'] ) ) {

		require_once 'utilities/MCAPI.class.php';
		$apikey = get_buyer_config_option('group-buying-theme-mc-api-key');
		$fieldname = get_buyer_config_option('group-buying-theme-mc-location-field');
		$api = new MCAPI($apikey);
		$api->setTimeout(5);
		
		$merge_vars = array( $fieldname => $_POST['deal_location'] );
		$listId = get_buyer_config_option('group-buying-theme-mc-list-id');
		
		$retval = $api->listSubscribe( $listId, $_POST['email_address'], $merge_vars );
		if ($api->errorMessage) {
			Group_Buying_Utility::storeMessage($api->errorMessage);
		}
		if ( !$api->errorCode && $_POST['deal_location'] != null || $api->errorCode == '214' && $_POST['deal_location'] != null ) {
			setcookie( 'your-selected-location', $_POST['deal_location'], time() + 24 * 60 * 60 * 30, '/' );
			wp_redirect( site_url() );
			exit();
		}

	}
}
remove_action( 'init', 'group_buying_subscription' );
add_action( 'init', 'group_buying_subscription_new' );