You can another recipient to your Xero invoices when emailing the agent/guest by adding a comma ',' followed by the email of the other recipient.
If the guest or agent always has another recipient included in the email, you can update their contact records in Xero to include the other recipient.
To do this, while in Xero go to Contacts > All contacts > click into your contact > Edit and then click Add another person.
Enter the contacts details including email address and click 'Include in emails'.
Now whenever you email an invoice for that contact in Xero, the other recipient will automatically be included.