While Australian Officials have been urging their citizens to shower together for many, many years now, this is a relatively new idea to me. Sharing meals saves energy. It makes sense - not all of the energy used while cooking goes into warming up the food, some of it goes to warming up air around the food. It makes much more sense to cook a 12 lb (organic, locally grown) turkey in one oven than to cook two 6 lb turkeys in two separate ovens in two separate kitchens.
So does "brown bagging it" by bringing lunch to the office, and as Jacqui keeps pointing out to me, you don't need to get a bag when you buy lunch if you're just going to throw it away moments later when you eat your meal. (Whole Foods even goes as far as to give you a 15 cent discount whenever you bring your own bag to the store, or when you take a plate instead of a plastic box for their salad bar.)
Of course, none of these options are that environmentally friendly if you end up driving to & from your friends' houses for dinner, or the restaurant, or shopping. It makes more sense to find a walkable neighborhood to live in in the first place and walk or take a bicycle. (My current neighborhood scores in the 60's and the one I want to move to scores in the upper 80's!).