Welcome aboard to the Community, @dcullen.
Let me share some clarification about emailing invoice in QuickBooks Online.
Currently, an option to identify who viewed the invoice is unavailable. You can only check the status of the transaction if one of the receivers open the emailed information from QuickBooks (see screenshot below).
Please know that your input about the feature is much appreciated. We continue to make improvements in QuickBooks Online, and this preference might be available in the future.
In the meantime, I suggest you visit our QuickBooks Blog to stay updated with all the changes to the product.
Also, I've attached some articles you can use to automatically send reminders to customers, as well as some additional information about emailing invoices:
Drop me a comment below if you have any other questions. I'll be more than happy to help. Wishing you a good one.