iOS 12.1 includes Group FaceTime and eSIM support, as well as 70 new emoji. Group FaceTime was supposed to be included in the first iOS 12 public release, but was pulled during beta testing.
The developer betas also hint at new iPad Pro models coming this fall. If so, they’re likely coming in October.
You need to be part of Apple’s developer program to get the developer betas. You can sign up at Apple’s developer website. If you’re already running iOS 12 developer beta you should see the 12.1 update when you go to Settings > General > Software Update.