The next upcoming update for iOS is now rumored to add Group FaceTime and dual SIM support to some phones. For some other, older models, however, only restricted forms of Group FaceTime are coming. Some others are not getting this feature at all.
https://www.notebookcheck.net/iOS-12-1-will-unlock-Dual-SIM-and-Group-FaceTime-but-only-for-newer-devices.346896.0.html