Adding to a Selection in Photoshop

To add to a current selection, simply press the Shift key and drag around the pixels you want to include with the regular Lasso or the Rectangular or Elliptical Marquee tool. You can also press the Shift key and click the area you want with the Magic Wand tool.

To include an area with straight sides in your selection, you can press the Shift key and click around the area with the Polygon Lasso tool. And although you may not have much need to do it, you can also press the Shift key and click with the Single Column or Single Row Marquee tool. I wouldn’t use the Magnetic Lasso tool to add to a selection; it’s way too cumbersome.

You don’t have to use the same tool to add to your selection that you used to create the original selection. Feel free to use whatever selection tool you think will get the job done.
Here are the steps to use for adding to a circular selection:

1. Make your initial elliptical selection. Select the larger circle with the Elliptical Marquee tool. Then press the Alt key (Option on the Mac) to draw from the center out.

2. To add the smaller circular area, first press the Shift key and then press the Alt key (Option on the Mac) to draw from the center out.

3. Drag around the smaller selection with the Elliptical Marquee tool.