Today, Apple has removed Group FaceTime from the latest iOS 12 and macOS Mojave betas. The new beta software was released this morning. This feature will certainly be released down the road, however an official date has yet to be announced.
Group Facetime is one of the key features of iOS 12 and macOS Mojave, Group FaceTime is designed to allow up to 32 people to chat together at one time via FaceTime audio or FaceTime video.