Magento – How to add the Company Name to the Billing Address information in a new order email

you need to create a plugin in your custom module ( the one that you used to create the custom attribute ), and have the code something like the following one:

namespace Package\Name\Plugin\Checkout;

class LayoutProcessor
{
    public function afterProcess(
        \Magento\Checkout\Block\Checkout\LayoutProcessor $subject,
        array $jsLayout
    ) {
        // Loop all payment methods (because billing address is appended to the payments)
        $configuration = $jsLayout['components']['checkout']['children']['steps']['children']['billing-step']['children']['payment']['children']['payments-list']['children'];
        foreach ($configuration as $paymentGroup => $groupConfig) {
            if (isset($groupConfig['component']) AND $groupConfig['component'] === 'Magento_Checkout/js/view/billing-address') {

                $jsLayout['components']['checkout']['children']['steps']['children']['billing-step']['children']
                ['payment']['children']['payments-list']['children'][$paymentGroup]['children']['form-fields']['children']['custom_attribute_code'] = [
                    'component' => 'Magento_Ui/js/form/element/abstract',
                    'config' => [
                        'template' => 'ui/form/field',
                        'elementTmpl' => 'ui/form/element/input',
                        'id' => 'custom_attribute_id',
                    ],
                    'dataScope' => $groupConfig['dataScopePrefix'] . '.custom_attribute_id',
                    'label' => __('Custom attribute label'),
                    'provider' => 'checkoutProvider',
                    'visible' => true,
                    'validation' => [
                        'required-entry' => true,
                        'min_text_length' => 0,
                    ],
                    'sortOrder' => 300,
                    'id' => 'custom_attribute_id'
                ];
            }
        }

        return $jsLayout;
    }
}

Hope this helps

There are few issue may be in your code,

getFirstName() should be getFirstname() as at db table field name is firstname.getFirstName indicated at the filed name is first_name and that is wrong

and getLastName() should be getLastname() as at db table field name is lastname().getLastName indicated at the filed name is last_name and that is wrong

Also you check the order 700590221 exists at system*.

For first case, you can check availability by $order->getId() give numeric value if order exits at system else give value false

Code

<?php
  $order = Mage::getModel('sales/order')->loadByIncrementId('700590221');
  if($order && $order->getId()) {
      $billingAddress     = $order->getBillingAddress();
      echo 'First Name: ' . $billingAddress->getFirstname().PHP_EOL;
     echo 'Last Name: ' . $billingAddress->getLastname();
  }
?>