What Should I do?

My very close friend is having a birthday soon - Several days before her birthday, six of us have planned to take her out. I recently have had some issues with 2 of the people planning to go out. I am trying to find a good way to "bow out" of this gathering. I will still bring my friend a gift and celebrate her big day another time. I just cannot bear to spend an entire evening being "fake" with the 2 people whom I am not happy with. What should I tell the birthday girl? Thank you.