Try and avoid anything too formal, shiny or stiff - casual is best!
Rather than matching outfits - choosing complementary outfits/colours is a better idea - for example, everyone in similar hues/tones.
If its an outside shoot and its cold - layers work best under your coat - you may be able to drop your coat between shots if it's not too cold.
Feel free to bring an outfit change to your shoot. However it is not necessary, you may stay in the same outfit for the entirety of the shoot. It just gives you a little more variety should you choose to use it.