Exposure correction to lighten photograph and make subjects visible and bright

"I'm not sure if the flash failed to go off or what, but this photo is terribly underexposed. It's so dark, the people at the far end can hardly be seen, let alone be recognised. Can you lighten this photo and put some life back into it. Also, there is a large reflection on the back wall that is very distracting. We intend to carry this picture in our monthly newsletter; how soon can it be fixed?"