Here's why:
* Pronoun Case: In this case, "you" and "she" are acting as the subjects of the sentence (the ones doing the action). The subject pronouns are "you" and "she", not "her".
* Pronoun Agreement: The pronouns should agree in number and case. Since "you" and "she" are both singular, you use the singular subject pronouns.
Example: "You and she are going to the party."
Let me know if you have any other grammar questions!