Navigate to Contacts > Customers.
Click on the Edit Contact link beside the appropriate customer.
Click on the Additional Contacts tab in the box on the left-hand side and hit the Add Contact button.
Enter the Title, Contact name and Contact email as appropriate.
Add more contacts by hitting the Add contact button again.
Hit the Save button to complete.
Navigate to Settings > Invoice Themes.
Click on the Preferences link.
Scroll to the bottom of the menu and select Enabled in the Invoice multiple contacts drop down box.
Navigate to the Sales > Invoices menu.
Three new drop-down boxes will be displayed. The Billing contact will allow you to choose the appropriate contact shown on the invoice. The Additional contacts box displays the contacts added from step 3. The Notifications box determines which contacts to be emailed.