Hello bin296,
That's a great question. There isn't a way to leave a group FaceTime call if the caller added you as a contact in the call. You can decline the incoming call to prevent joining. You can learn more about Group FaceTime calls via the following article.
Use Group FaceTime on your iPhone, iPad, and iPod touch
Apple appreciates all feedback regarding their products, services, and features. If you would like to provide feedback about Group FaceTime, you can do so via the Product Feedback website. This would ensure they're able to review the feedback you provide.
Another option that's available is to block the number, Apple ID, or contact the FaceTime call is coming from. You can block the caller using the steps listed below. You can also find these steps outlined in the following article.
- Tap Settings
- Face Time
- Blocked Contacts
- Add New
Block phone numbers, contacts, and emails on your iPhone, iPad, or iPod touch
Take care!