Install if Your Testing the iOS 12 Beta
Are you testing the iOS 12 beta on your iPhone? If so you might want to install the macOS Mojave beta on your Mac to test out the new features side by side.
If you want to get full use out of the new syncing features in Apple's apps and use iOS 12 to the fullest, you will need macOS Mojave installed.
Right now Group FaceTime isn't available on the Mac, but it will likely come to a future beta and when it does you will be able to test out that feature.
You can already use the Continuity feature in macOS Mojave to take a picture or scan a document with your iPhone that automatically appears in an app on your Mac.