In order to add an additional email to a client's Quickbooks account it needs to be added in 2 places:
the recurring invoice notification which is already set up
the client profile itself to have it included in future new invoices as well
To add it to the already set up recurring invoice, go to Quickbooks > ⚙️ > Recurring transactions and search for the client's name. Once located, click on Edit to open the recurring transaction. Lastly, add the email address into the email section and press Save template.
To add the email address to the client's profile, go to Quickbooks > Sales > Customers and search for the client name. Once located, click on the profile and then on Edit. Lastly, add the email address into the email section and press Save. The email address needs to only be added to the Customer in Quickbooks, not to the Project.