A few things come to mind:
1. Experiment with the filter Christoph mentioned in order to become familiar with it.
2. Use Camera Raw and experiment with the features specifically in the Lens Correction section.
3. Maybe experiment with the Spherize filter with the Horizontal Only setting.
4. Consider shooting future photos from further back with a lens with a higher focal length. I wouldn't suggest shooting a group of people at a focal length lower than, say, about 30mm on a 1.6x crop camera or 50mm on a full frame camera.
I mentioned item 4 because you may find it will be difficult to correct wide angle distortion well enough digitally to where the people will look natural.
-Noel