How to split the expenses for a group Bat Mitzvah celebration event. The celebration is for 4 girls, 2 sisters and 2 non related girls. Should it be split in 3, per family, or equitativly between the 4 participants?

A fair and pre-agreed settlement should be reached between all partners of the joint event. Any arrangement that everyone agrees on in advance is halachically acceptable. A reasonable arrangement would take into account that each family would pay for its own expenses as much as it could be calculated. For example: If it is an event for the families of the Bat Mitzvah girls, then each one of the three families will pay for their own family. If it is an event for the classmates of the Bat Mitzvah girls, and the two sisters are in separate classes, then the payment will be according to the participating classes. As a result, in this case the family of the two sisters will pay for two classes. If it is possible to calculate the precise expenses per participant, and you know exactly who will participate, that is excellent. If that is not possible, then the agreement should be based on assessment.