I will use equivalent FL so we don't need to care about body and sensor size.
Unless you have absolutely no space to back up, I suggest use no shorter than 30mm FF equivalent for group shots. Use longer FL if you can, like 35mm or 50mm. The problem with short FL is perspective distortion: People in front row will appear noticeablely bigger than people in back row. People in middle appear bigger than people on the outside. People on the outside appear to be wider (fatter) than they actaully are ..... All in all, it doesn't give a natural look.
I agree, with only comment being that I have found 28mm (135 format equiv.) to be safe, having covered weddings in three different formats (APS-C, 135, 645)

