How can I tell her nicely?

For prom, I'm going with my best friend, however another semi-friend doesn't know this and thinks that she is going with my best friend, even though she hasn't actually asked her. This semi-friend doesn't really have anyone else to go with but has said she's interested in a limo with loads of other people (which hasn't been booked) How can we tell her that she can't come with us, without hurting her feelings? Thanks :)