@Stephanie1215 You could send the guest a message- "Hi XX, it is imperative that you amend the booking to include the exact number of guests who will be arriving with you, as unregistered guests will be denied entrance. Please do this ASAP."
As she hasn't responded to your other messages, she likely won't respond to this one, but at least there will be a record of you having conveyed this information.
I also smell a rat.
Make sure when you talk to CS that you tell them you have a feeling the guest is misrepresenting her intentions and intends to throw a party, as this is front and center right now after the Orinda shootings. And that she hasn't responded to any of your messages. Ask them to cancel the reservation penalty-free if the guest doesn't respond appropriately.