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.
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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.
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To do this, while in Xero go to Contacts > All contacts > click into your contact > Edit and then click Add another person.
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Enter the contacts details including email address and click 'Include in emails'.
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Now whenever you email an invoice for that contact in Xero, the other recipient will automatically be included.