You have a couple options. Ignore it and go on as if nothing happened. Be bratty about it and exclude her from something you do with friends.
Let's be honest here you all are not glued at the hip and where one goes you all go. You each have your own lives within the group and outside of the group. You are each free to have personal time away from the entire group or part of the group.
It was none of your business who she was going to the beach with. It was rude to ask. You could have said cool have a good time and let it go. You sound jealous more than you sound as if you feel not good enough. Some times people just want to spend time with people who fit their mindset at the moment. It has nothing to do with saying you are less. Maybe she had a specific reason for wanting to spend time with them because they offer something you don't, however, another time you fit the bill and they don't.
Take the high road and don't behave poorly.