After 14-year-old Grant Thompson discovered that a caller could listen in on another FaceTime user while the device was ringing, Apple quickly disabled Group FaceTime until it could resolve the problem (see “Apple Disables Group FaceTime to Block Glaring Privacy Hole,” 29 January 2019). Apple later addressed the bug on the server side (see “Apple Fixes Group FaceTime Bug; Promises to Improve Bug Reporting Process,” 1 February 2019), but said additional software updates would be required to re-enable Group FaceTime.
Those updates are now here in the form of iOS 12.1.4 and macOS 10.14.3 Supplemental Update. You can obtain iOS 12.1.4, which weighs in at 89.6 MB on an iPhone X and 65 MB on a 10.5-inch iPad Pro, in Settings > General > Software Update or through iTunes. The macOS 10.14.3 Supplemental Update weighs in at 987.7 MB, and you can install it from System Preferences > Software Update.
Don’t expect any new features from these updates, as they’re aimed at resolving the Group FaceTime bug. However, Apple took the opportunity to fold in a few other security fixes, providing a total of four in iOS 12.1.4 and three in macOS 10.14.3 Supplemental Update. (Interestingly, in the security notes, Apple credits both Grant Thompson and Daven Morris of Arlington, TX for identifying the FaceTime bug—Morris hasn’t previously been mentioned in any press reports.)
Unless you’ve been missing Group FaceTime, it would be wise to wait a couple of days to make sure these updates don’t introduce any new issues. Remember since Apple fixed the bug on its servers, this update isn’t necessary to protect yourself from the exploit, merely to re-enable Group FaceTime.