When sending an invoice, you're able to send to other email addresses by adding them in the 'To' section of the Send Email form, separated by a comma.

It can be done the same way in the App, when you go to send the invoice from there. In the invoice, you go to Email, and then when inside the email window, you can either add another saved Contact that already exists in your Formitize account (this is good if you have multiple contacts under the same billing name), or simply type in the extra email address in the address bar, separated by a comma.

You can also set up extra email addresses directly to a contact, so that invoices will always send it to more than one person. This can be done in the Web Portal, by opening up the Contact card and editing the Billing details. The easiest way to make sure it sends to all the emails every time is to add the secondary addresses in the same line as the main billing email.