Speaking about new iOS 12 features arriving later this year via a software update, there is speculation that the new developer beta might include a batch of fresh emoji that is expected to arrive via a future iOS 12 update. The build number for the update is 16B5059d and it weighs in at 2.77GB.
Keep in mind that the idea behind the developer beta is for developers to see whether their apps run properly with the changes made to the last stable version of iOS (which is iOS 12 in this case). The iOS 12.1 developer beta is not meant for consumers and even if you're dying to use Group FaceTime, you might want to wait for the official OTA version of iOS 12.1 to hit your iPhone.
Meanwhile, the day after watchOS 5 was released, Apple has made the watchOS 5.1 developer beta available. The latter's build number is 16R5559d.