Not only are Layer Groups great for organizing your layers, you can also use them to mask the contents of multiple layers at a time. With the Group targeted in the Layers panel, click the Add Pixel (or Vector) Mask icon from the Masks panel. This mask will control the visibility of all layers within the Group.
To paste content (from the clipboard) into a Layer mask, Option (Mac)/ Alt (Win) -click the Layer mask icon on the Layers panel and then select Edit > Paste.
If you have an active selection in your document (marching ants) and have content on the clipboard, selecting “Paste Into” will paste the content from the clipboard into your selection - and automatically convert the selection into a Layer mask.
To temporarily disable a layer or vector mask, Shift-click the mask icon in the Layers panel. For a Layer mask, simply clicking on the mask thumbnail in the Layers panel will enable it. For a Vector mask, you must shift -click the thumbnail again to enable it. You can also Control (Mac) / Right mouse (Win) -click on the mask in the Layers panel and choose to Enable or Disable the mask.
To view a layer mask, Option (Mac) / Alt (Win) -click on the mask thumbnail in the Layers panel. Tapping the “\”(backslash) toggles the display of a layer mask on and off (as a red rubylith overlay). (Looking at the Channels panel, you can see that this shortcut toggles the channels visibility.)
Command (Mac) / Control (Win) + “\” (backslash) targets the layer mask in the Layers panel. Command (Mac) / Control (Win) + 2 targets the layer.
Drag a layer or vector mask thumbnail in the Layers panel to move it from one layer to another.
Option (Mac) / Alt (Win) -drag a layer or vector mask thumbnail in the Layers panel to create a copy of the mask.
Option (Mac) / Alt (Win) + Shift -drag to create copy of a layer mask while simultaneously inverting the mask. (Note: this shortcut does not work with a vector mask - in order to invert a vector mask, select the path with the Direct Selection tool and click the “Subtract From Shapes Area” icon in the Options bar.)
To delete a mask, target it on the Layers panel and click the Trash icon on the Masks panel. If you prefer to click the Trash icon on the Layers panel, or drag the mask thumbnail to the Trash icon at the bottom of Layers panel, adding the Option (Mac) / Alt (Win) key will bypass the option dialog box. You can also Control -click (Mac) or right mouse click on the layer mask and choose discard from the context sensitive menu to bypass the dialog.
To add a mask to a layer, click on the layer or vector mask icon in the Masks panel. If you prefer to use the Layers panel “Add Layer Mask” icon, click once to add a layer mask, click again to add a vector mask, or if you want only the vector mask, Command (Mac) / Control (Win) -click the mask icon. To add a layer mask which automatically hides the selection (as oppose to reveals it as it does by default), Option (Mac) / Alt (Win) -click the icon. Of course with Photoshop CS4, you can always invert a layer mask using the new Invert button on the Masks panel!