If you want to leave the call, hit the red button towards the bottom-right of the screen. If you need to add others from there, just swipe up, tap 'Add Person' and enter their info. This will add everyone in the group chat to the video call.
The easiest way to start a Group FaceTime call is to open an existing group chat in the Messages app, tap on the group of contacts you're talking to at the top of the screen, then hit the FaceTime icon. The relatively new Group FaceTime feature lets you virtually hang with your friends fairly seamlessly.
One-on-one FaceTime works well, but it took Apple a while to figure out how to make it work with groups.
