Important : As always with new features, FaceTime requires that all participants have iOS 12, macOS Mojave or watchOS 5 installed on their devices in order to participate.
Groups FaceTime via App
Of course, the first impulse to start a group call is to open the FaceTime app. Then tap the "+" symbol in the upper right corner. Now enter the telephone numbers of the participants in the input field "To:" or select them from your contacts via the "+" to the right. Repeat the process until you have entered all the participants and then tap the audio or video option. Then the connection to all participants (as far as possible) is established.
Group Calls via iMessage
You do not have to resort to Apple's FaceTime app to make FaceTime calls. This also applies to groups. To do this, just open the group you want in the Messaging app and tap the top of the group photo. Underneath now appears the option "FaceTime". A hint on this will give you the opportunity to choose between an audio and a video call.
Groups FaceTime via iMessage has a big advantage. In this case, the call will be displayed within the group, allowing group members to board later or quickly reconnect after a disconnect without having to manually add the person back in.