Now back to this particular situation, that is quite ugly from the Sony JPG engine?! There is little you can do with that, as the detail is already destroyed by the mosaics. There are a few techniques available to try to blur contrast points around the noise a bit, then do an overall slight reduction, but when you sharpen back, there won't be anything left really to sharpen, at least not in the crops you posted from your sample.
It sounds like the camera was shot in JPG mode only, so there are no raws to go back to. I have no idea if it is possible, but is there any way to coordinate a re-shoot with everyone? This way you can go with raw, and also recompose the scene a bit differently, so that people are wearing footwear, and there aren't decorations coming out the back of peoples' heads. These are easy traps to fall into though, I still do that often. I go back to a pic, and go "WTH?" because there are these things in the background placed in very terrible places in relation to the heads.