Feature to add people in conversations on FaceTime.Photos: Macrumors.
In response to user feedback, the Apple Support support account on Twitter only instructs them how to use Group FaceTime when "making a call with at least two people".
In addition to limiting the feature of adding members when chatting with FaceTime, iOS 12.1.4 update also has a series of other serious errors such as not accepting sim, losing wifi connection, 4G.