From the Invoices or Invoice Items tabs in your Optix dashboard, you can access all invoices created, and have the ability to filter these invoices by a particular user. You can do so by typing their name in the search box at the top right of the page.
Another way to do this would be to:
Navigate to your Users and select the user
In their account panel, navigate to the Invoices section to find all invoice records or create a new invoice via the '+' button
Need more info on Invoices?