Is there a way to send simultaneous Email Alerts to people, including those who are not on my contact list?

Yes, you can.

Apart from adding contacts already stored in Zoho Invoice as Email Recipients, you can add the email addresses of people whose details are not stored in Zoho Invoice.

Enter the email addresses separated by a comma under Additional Recipients. Click Save to finish. Henceforth, if you select this particular Email Alert, all the contacts listed here will receive an email when the workflow is triggered.

Note: The maximum number of additional recipients is 10.