https://www.cnet.com/news/apple-hasnt-quite-got-iphone-group-facetime-right-after-fixing-bug/

Apple Support confirmed this limitation on Twitter, after noting that you have to update to iOS 12.1.4 to use Group FaceTime at all. The feature was temporarily disabled after a Fortnite-playing teen discovered the bug.

"Group FaceTime calls need to be started with at least two additional users in the FaceTime app," it wrote.