I recommend that the parents both wear black or both wear white. I like to keep it simple for newborn photos. Since the baby is so tiny and the focus is on the addition to the family, we want to keep it simple.
In addition to wearing either all white or all black, I recommend bringing a backup shirt in case baby has an accident when are doing any shots while the baby is nakie.
As far as what baby should wear, you can have them in an all-white onesie. Most of the time baby will be nakie or swaddled in wraps or in an outfit that I have. If you have something sentimental, please bring it. I have a variety of hats and outfits.