Earlier this month, Apple pushed an update on iPhones and iPads, dedicated to fixing a bug, which was transmitting user audio, and sometimes even video, to the other users before they answered the call. This was happening when a third person was added into a group FaceTime call.
The bug was found by a teen and his mother who informed Apple about the same, only to be ignored until the news of the bug was picked up in the news.
However, before Apple released a patch to fix this issue, temporarily, Apple disabled the group FaceTime feature from its end.
The feature was later available to all when the fix was rolled out via an over-the-air update. However, according to some users on the MacRumor forum, since the update, two people on a FaceTime call cannot add a third person, with the button to add being greyed out. The same issue was also reported by ZDNet.
Apple Support has recently confirmed this limitation in a post on Twitter saying that you have to update to iOS 12.1.4 to use Group FaceTime at all.
That's good. Also, note that Group FaceTime calls need to be started with at least two additional users in the FaceTime app. Meet us in DM if you're still having an issue with more users: https://t.co/GDrqU22YpT
" Apple Support (@AppleSupport) February 15, 2019
It is still unclear if Apple is working to fix this issue with the upcoming iOS 12.2 update. We have dropped an email to Apple to know more about this.