if ($this->getCustomerIsGuest())
and add the following code into the else statement:
$customerGroup = Mage::getModel('customer/group')->load($this->getCustomerGroupId());
if ($customerGroup->getEmail()) {
    $copyTo[] = $customerGroup->getEmail(); // getting value from your custom field
}

if ($this->getCustomerIsGuest()) {
    $templateId = Mage::getStoreConfig(self::XML_PATH_EMAIL_GUEST_TEMPLATE, $storeId);
    $customerName = $this->getBillingAddress()->getName();
} else {
    $templateId = Mage::getStoreConfig(self::XML_PATH_EMAIL_TEMPLATE, $storeId);
    $customerName = $this->getCustomerName();
    $customerGroup = Mage::getModel('customer/group')->load($this->getCustomerGroupId());
    if ($customerGroup->getEmail()) {
        $copyTo[] = $customerGroup->getEmail();
    }
}