There was a request that became a feature to be able to use a layer group as the base of a clipping group, (http://feedback.photoshop.com/photosh... ) but I was facing the opposite need yesterday: be able to group layers that are being clipped in a clipping mask.
My goal was to mask several clipped layers, and organize my layers panel; there is no way to group layers that are part of a clipping mask. (The add layer group icon is not greyed out, but dragging the layers to the group removes them from the clipping mask)