You don't say where within the email you want the company name value to appear, but the module /includes/modules/create_account.php handles that email formatting and output.

You'll want to make your edits in your template-override folder, so if your template name is mytemplate you need to:
  1. Copy /includes/modules/create_account.php to /includes/modules/mytemplate/create_account.php (creating this folder if it doesn't already exist)
  2. Make your edits to /includes/modules/mytemplate/create_account.php