Apple earlier this week released a new beta of iOS 12.2 to developers and public beta testers, but that beta does not include the same Group FaceTime bug fix that was introduced in today's iOS 12.1.4 update.
Apple's Group FaceTime servers are back online, but because the issue has not been fixed in iOS 12.2, Group FaceTime is not working on beta devices.
We have been testing Group FaceTime since the servers came back online, and while we can get calls to work between multiple people on devices running iOS 12.1.4, calls do not go through on devices running iOS 12.2 beta 2.
Developers and public beta testers will need to wait until the Group FaceTime bug fix is added to the next iOS 12.2 beta before being able to use the feature.
Group FaceTime is also unavailable on devices running iOS 12.1.3 or earlier, and will remain unavailable until the devices are upgraded to iOS 12.1.4. The same goes for Macs that are not running the newly updated macOS 10.14.3 update. Group FaceTime on Mac doesn't work on Macs running the older version of macOS 10.14.3 or the new macOS 10.14.4 beta update.