It is actually new with iOS 8 because with my 3 devices, 2 are on iOS 7 and one of them is on iOS 8. Only the iOS 8 device lets me add a person to an active call, and the iOS 7 devices can't be in a FaceTime conference call.
The iOS 8 device will put one on hold, it gives a "Merge Calls" button, which stays greyed out because iOS 7 doesn't support it.