There’s a major bug in FaceTime that gives callers access to your microphone and/or video camera without granting permission. Rene Ritchie has the details, and Apple has issued a statement that this bug will be addressed “later this week.”
In the meantime I’d recommend going to your Settings app and turning off FaceTime altogether. This is really about as bad as it gets.
Update: Looks like Apple has turned off Group FaceTime? Good call.