Apple recently released their iOS 12.1.4 software update and the update was designed to fix the Group FaceTime bug that had some serious security issues.
Whilst the iOS 12.1.4 software update did fix the security issues with FaceTime it would appear that there are still some problems with Group FaceTime.
According to MacRumors not all the functions of Group FaceTime have been restored, Apple is apparently aware that there are still some issues.
@AppleSupport Group FaceTime doesn’t work when I hit add person on iOS 12.1.4
— DaJé (@DaJe) February 15, 2019
As yet there are no details when all of the functions will be restored to the Group FaceTime, as soon as we get more information, we will let you guys know.