Apple has been criticized for the delay in responding and has promised to improve procedures.
The bad news was you couldn't use group FaceTime for more than a week while Apple worked on the fix. The arrival of the new version of iOS comes just two days after a pair of congressmen sent a letter (which you can read here) to CEO Tim Cook that included a list of questions they'd like him to answer in the wake of the bug. It is likely that Apple will keep Group FaceTime disabled for devices that are running earlier versions of iOS than iOS 12.1.4, though. The changelog mentions "important security updates", and those include a fix for the Group FaceTime bug that was recently publicized.
Apple has released an update for its flawed FaceTime video calling feature that allowed users to listen to and watch other callers through their devices, even if they didn't give their permission. As Apple's system status page points out, the group FaceTime servers are now back online and you can once again make group FaceTime calls- but only after you update.
Apple says it has fixed the Group FaceTime issue, discovered by 14 year old Grant Thompson of Catalina Foothills High School, in all iOS devices including the often forgotten iPod touch.
Apple plans to contribute to his college fund and will also pay a bounty to him and his family for reporting the bug. Apple has issued iOS 12.1.4 and macOS 10.14.3 to users through their software update tools a little later than expected.
While you're there, you can choose to automatically install updates.
On supported devices, simply open Settings - General - Software Update and follow the onscreen instructions.