It won't be an easy job either - rectilinear UWAs have usually quite a flattened frame throughout the center, then the ends bend a bit more. The Sigma 10-20 in particular does a fine job of keeping horizontal and vertical lines in alignment and without distortion through the main part of the frame, but then bends significantly in the two far ends. So a basic barrel correction unfortunately won't do the trick. You may have to use a grid overlay to manipulate just the left and right edges of the frame, or apply barrel correction as a new layer then erase the center portion to restore the original shot.
For the future, when using UWAs as portrait lenses for group shots, try to leave a little dead space at the left and the right, so it can be cropped out and avoid stretching anyone!