It's actually pretty easy. Groups are created by dragging an icon in between two other groups (drag up to start the move on tablets):
Simply organize your icons into the groups that you want. If you're looking to make the icons smaller you can do that by right-clicking (press and hold or swipe down on tablets) on the icon and select Smaller:
Finally to name groups you have to zoom out (Ctrl+Scroll or Pinch Zoom) and then right click (swiping down) on the group and Select "Name Group":
There you have it! A quick and easy way to organize your apps on the "Modern UI".