If you're working on a group project and you're accused of plagiarism because of another student in your group potential of doing so, you certainly should not, under normal circumstances be culpable for that other student's misconduct. That being said, regrettably, you can never depend on the school, the professor, or the people in the conduct office to necessarily do the right thing. In terms of what needs to be done to try to make certain that you're not found responsible for an allegation of that nature, you have to take the necessary precautions.
You have to prepare the best possible response and potential defense as early as possible in the process. Often schools would invite a meeting between the student or the professor. Other schools would say forward it directly to the conduct office for subsequent steps to be taken. You cannot depend on the school to do the right thing. You have to be prepared before you go into any meetings or have any conversations with either the professor or the conduct office or say other parties that may be involved.
Having an experienced academic misconduct attorney advisor is going to be your best ally in helping to understand and navigate the process and they should be involved as early as possible in the process if you're accused of plagiarism.