I'm here to assist with your concern about adding another cashier for your invoices in QuickBooks Online (QBO).
Before we proceed, I'd like to confirm if you're using either Location or Class tracking to list your cashiers. Either way, you have the option to add a new one to the list. Both have the + Add new option when you click the drop-down menu.
If you're referring to something else, I'd like to ask for additional details on it.
I'll leave these articles for your reference:
I'll look forward to hearing back from you. Tag me in the comments below, and I'll get back to you as soon as I can.