No, I'm not the outsider in my office at all. Half of our department just went out to lunch together and no one even thought to invite me! I sit right there in cubeworld with the rest of them so it's not like I'm hard to remember or anything. It's even a restaurant I was really craving right now. I'd go on my own, but that would be ackward. Besides, I carpool and this isn't my week to drive so I don't have a car.
Ugh, I feel your pain. One time when I was relatively new to the office (had been there for a month or so) EVERYONE in my department went out to this hip restaurant (and when we went out to eat it was always paid for by the company- loooove working in sales! lol) I had been dying to go to but couldn't really afford it on my own- and they didn't even say a word! And it wasn't like they needed me to stay behind and answer the phones (I was an assistant so that was part of my job) because other assistants cover each other during lunches. THEN when they got back someone actually asked me why I didn't come! They obviously just didn't even think to ask me, how awkward is that???
I am sorry for you, though, it totally sucks. But, it really is just one of those "business world" things where you don't always get asked to lunch, better to just accept it and eat by yourself on these occasions and work to form a regular/semi-regular lunch group of your own so this doesn't happen very often in the future!