If you have been using shapes to space your parts apart, you probably don't want those shapes showing up when you save the image. There also may be other parts, wires, or systems that you don't want to show in the final drawing.
Step 1 is to create the new group. You will want to name it something that is easily understood, such as "Red Wires" or "Smoke Detector Spacers".
The next step is to apply the group to the item by selecting the shape, system, part or wire, and changing the group. Thats it! You can then go back to the group and hide or show it by clicking on the eyeball.