You'll want this method if you're going to change the foreground but not the background. Changes to the background still require re-copying the background the masked image at the top. If the opposite is true (you're going to be changing the background a lot, and the content not much), then you are better off simply merging the layer group into a single layer.
Make a clone of your background (or whatever you want to show behind the thing you're masking)1. Then add a layer mask to this new layer that is the opposite of what you want. That is, if you want something in the other layers to show(opaque), make that area of the mask black (for transparency of the background) and if you want them to be hidden(transparent), make that area of the mask white(opaque background above them)
If you have a pre-made mask, simply apply it as normal, then with the mask selected go to Colors > Invert.