For Family/Group Photos, dress in the same solid colors; your outfits doesn't have to match completely, but make sure your group colors collaborate, this will help to avoid colors from clashing.
Try to find out from your Photographer where the location will be, because the environment of the shoot could also become a big clash. If you are outside where there are lots of greens and browns, try to avoid green and brownish colors. If your photo shoot is on a beach try to avoid colors that match the sand.
Please do not get discourage when choosing an outfit for a shoot,look at other photographs for inspiration, look at how their outfits collaborate with one another. Photo portraits are about people, but if you chose the right outfit/s it could be your best portrait yet.