How to tell my friend she is not invited

I have a planned vacation outside the country with my family next month. I mentioned this to my friend and she told me she'd like to go with us. She has not met any member of my family and i have only known her less than a year. How can I tell her in a way that would not hurt her or make her feel bad that me and my family cannot invite outsiders and we prefer to have the trip by ourselves? Any advice would really help. Thanks

Let her know the reservations were made with a head count prior to you knowing she was able to come.