Von Photographylife:
How does Group-area AF work? Basically, within the viewfinder you see four focus points, with the fifth one in the middle hidden. You can move all four focus points by pressing the multi-touch controller on the back of the camera (ideally, you want to stay in the middle, because the focus points in the center of the frame are cross-type and the most accurate). When pointed at a subject, all five focus points are activated simultaneously for the initial focus acquisition, with priority given to the closest subject. This differs from the the Dynamic 9 AF mode quite a bit, because D9 activates 8 focus points around the center focus point, with priority given to the chosen center focus point. If the camera fails to focus using the center focus point (not enough contrast), it attempts to do it with the other 8 focus points. Basically, the camera will always prioritize the central focus point and only fail-over to the other 8 if focus is not possible. In contrast, Group-area AF uses all 5 focus points simultaneously and will attempt to focus on the nearest subject, without giving preference to any of the 5 focus points.