Hi there, networkingza.

You'd be glad to know that this is possible in QuickBooks Online. Allow me to walk you through the steps.

1. Select Sales tab and select Customer tab.
2. Select New customer to create a new customer or select the customer then click on Edit. (if the customer is already created).
3. In the Email field, type in the email addresses separated by a comma (,) and space.
   Example: test_email@hotmail.com, email_test@yahoo.com
4. Select Save.

Let me know if there's anything else you need with QuickBooks Online. I'm here to help.