Inorder to send invoice to multiple recipients, invoice should be addressed to a contact instead of primary carer.
Xero users please see the note at the end of this article.
Once you have created invoice for Client's contact, then you can add comma separated email into email field for the contact, as shown below. The emails should be separated by a comma e.g. firstname.lastname@example.org, email@example.com, firstname.lastname@example.org
Finally, go to the invoice and click on the send email button to send the invoice to multiple recipients.
Warning for Xero users: When importing invoices to Xero, if multiple email addresses are used the import will fail. A suggested workaround for this would be to send the invoice to a contact with multiple email addresses then edit the invoice, changing it to be directed to one recipient.