Hi, dcullen.

As mentioned by my peer above, the option to check who viewed the invoice is unavailable in QuickBooks Online. I appreciate you explaining how it would help you out. While that feature isn’t currently in our development road-map, we are working on big improvements that will be helpful for the majority of our customers, and you’ll see those rolling out soon.

I recommend visiting our QuickBooks Blog website for updates. You can bookmark the following link: https://quickbooks.intuit.com/blog/.

While we can’t build exactly what you’re after, as a workaround, you might want to send the invoice individually so you'll know who viewed the transaction. 

For your reference, you can visit this link: You can do WHAT in QuickBooks? Invoicing. This will give you more ideas on what other information you can add to your invoice template as well as tracking work performed using the Google Calendar in QuickBooks Online.

If you have any other concerns, please let me know. I'd be more than willing to lend you a hand.