If its a daily lunch or dinner, its a harder deal, but there are all sorts of excuses for a single time thing.
One that usually works is that you just basically aren't that hungry. Just a polite "you know what, I'm actually not hungry right now, but I might grab something if there are leftovers."
or sometimes if you know there is going to be one, spend the day kind of mentioning that you aren't feeling well. Mention it to a co-worker or something like that early inthe morning and then when lunch comes around you can just say "i'm still not feeling that well so i think I'm just going to have some water and hope I feel better by the end of the day." (I used this one on Friday for a birthday party with all sorts of cake and snacks and nobody thought a single thing except remorse) and then watching them eat and me not eat made me feel SO good!!!
i think those work best, at least from experience. it'll get easier and more excuses will come as the time goes by!
good luck girls, we're one day closer to being thin!