My friend is going to be mad at me but...

For my 18th birthday I am having a hotel sleepover with a few of my friends. The problem is that I can't invite everyone because it cost too much. The more people, the more money. So I am only going to invite 5 girls. And the problem is that I won't be able to invite her because I already have 5 friends in mind. Ok I know it sounds mean to leave her out but I can only invite 5 girls and these are my close friends. So it's kinda hard. She's been talking to me about my birthday and she wants to hang out with me for my birthday. And I feel so bad for not inviting her. Idk how to tell her? Should I? Because she's going to find out sooner or later because my friends are going to post pictures of us on their Instagram.... so what should I do? She's going to be very upset if she's not invited, but like I said I can't only invite 5 girls.