Invoices can be resent to registrants via the Payments report:

You can then use the search box to locate the invoice by typing in the invoicee’s name or the invoice reference number. ┬áYou can then click the View icon (magnifying glass) to view the invoice prior to resending it:

Then, from the Invoice Actions dropdown you can click “Resend” which will resend the invoice to the original invoicee:

The invoicee email will be confirmed on clicking the resend button. ┬áIf you need to resend the invoice to someone else you will need to edit the invoice via the “Attendees” report.