You can send your PDF invoices to several e-mail addresses at one time. You simply enter a comma between each e-mail address (no spaces) and the invoice will be delivered to all e-mail addresses.
Example that works: (separated by commas without spaces)
Examples that do not work: (Separated by commas and spaces/semicolon)
email@example.com , firstname.lastname@example.org , email@example.com
firstname.lastname@example.org ; email@example.com ; firstname.lastname@example.org