I have a dilemma with friends in groups for an upcoming school trip?

The Chorus and Band students at my school get to go to Islands of Adventure this May. We were recently asked to put together groups of who we wanted to hang out with in the park all day. So I put together a group of three, myself, my BFF since 5th grade (Who is in Band) and another friend in my Chorus class. Everything looked like it was going to work out, until a friend that I planned on inviting into the group that I was putting together invited me into her group. She told me that she had 4 people in the group and had room for one more (Our limit is five). She is a friend that I used to have two classes with last year and who used to be really close with me and is in Chorus just like myself. When I was about to tell her that I already had a group, she wrote my name down on the group sign up slip that we have to give to the teacher in order to confirm our groups. Now I am stuck in two groups, and don't know what the heck I'll do. I don't want to bump off my BFF, but I also don't want to hurt the feelings of my other friend.

How can I sort all this out? Please help!