Apple has disabled group Facetime because of a bug that allows others to listen even before they accept the call, without their knowledge. This happens when you –
1. Initiate a FaceTime call with someone.
2. While the call is ringing, swipe up from the bottom of the display.
3. Tap on the “Add Person” button.
4. Add your own phone number when it asks for the number of the person to add.
Not only the caller can listen into your conversation, but this bug also broadcasts video, creating huge privacy implications.
The only way to avoid this bug is to disable FaceTime. Read here to see how you can disable FaceTime in your device.