Apple's Group FaceTime woes don't seem to be ending just yet. It was discovered a few weeks ago that there was a security vulnerability that was incredibly easy to exploit, which would allow you to hear a recipient's audio before they answered the call. Apple was quick to react, shutting down the service until a fix would arrive, and it did arrive the next week in the form of iOS 12.1.4.
Unfortunately, while Group FaceTime is working again, the feature seems to be somewhat handicapped, as users are finding that you can't add others to an existing FaceTime call. As of right now, a FaceTime call will need to be started as a Group FaceTime call in order for it to work.
Apple's page for Group FaceTime does still show the ability to add a person to a Group FaceTime call, although it doesn't explicitly say that you can turn a regular FaceTime call into a Group FaceTime call. The page has been updated for the recent iOS 12.1.4 patch.
Hopefully, this is a temporary measure and there's a fix on the way.