rsvp -- Guest we invited responded "yes" and wrote in "and guest"

Ooo tough.

I would have you and your fiance call her together on the phone, or set up a brunch meet. Just be upfront about not allowing people to add guests, that the list has already been finalized. Maybe inquire who it is she is wanting to bring, since she only wrote "Guest". Has everyone else RSVP'd yes? We got a couple of cancelations so we have had spots open up.

Regardless of her dating history, are you sure she isn't in a relationship now? Brides and grooms don't get to decide the seriousness of someone else's relationship, so it's decent to invite all guests' significant others (regardless of their dating length). If she's truly single and just searching for a plus one, you can reach out and say, "Sorry for any confusion, but due to limited space at our venue, we cannot accommodate any plus ones. Please let us know if you have any questions."

View Quoted Comment I am positive she is single. We verified any unknown relationships prior to finalizing our guest list. She just wanted to bring a date. When I spoke to her she didn't even have a person in mind.

My response. She replied, said ok, I will come alone. Said she is looking forward to it.

View Quoted Comment That was perfect and straightforward

View Quoted Comment Thank you!

View Quoted Comment

Very diplomatic--good job!

Just tell her that you only invited X people because that's what you could afford. If she insists on bringing someone else, she can pay Y dollars.