@Lisa6735 By saying no. This guest is probably being dishonest about her intentions.
In fairness to the guest, your house rule ("Hosting company while staying is prohibited without prior consent") could have been less equivocal. Saying that unregistered guests are sometimes allowed only encourages guests to try their luck, and then feel miffed when the consent isn't granted. Better to have a 100% firm stance in your rules - no extra visitors, ever - but keep it to yourself that you might privately make exceptions for special guests who earn your trust.
Whenever possible in customer service, I try to spin a no as a yes to something else, so I might recommend a favorite nearby location to celebrate. I see you're between Fertility and Smoketown, which sounds like a blast 🙂