Select "Layers" in the left bar, then scroll down to the layers you want to link.
Click on the blank box directly to the left of the name of each layer you want to link. A small link image will appear. Continue until you've selected all the layers you'll mask. Click the arrow in the upper-right corner of the left bar, and select "New Set From Linked." The layers are now combined into a set, and they will all condense into "Set 1."
Click on "New Mask Layer" at the bottom of the mask bar to create a mask. Now all properties you apply to the mask will apply to all layers in Set 1.