I recently threw my first full-on 10-person dinner party after moving to a new city - a momentous affair! About an hour before guests were due to arrive, I realized we'd gotten rid of some furniture in the move and I was now about three seats short. All I can say is, it's surprising what works as a "seat" when you're desperate.
Here are a few quick fix solutions my husband and I thought of:
1. Call in the Troops! - We sent out a desperate text asking a few of our guests if they might bring a chair or two along with them. These were our closer friends who we knew (or hoped!) wouldn't feel too put out by this request.
2. Milk Crates - We have a few of these still hanging around from our college years. As a seating option, they work just as well now as they did back then.
3. Exercise Balls - These are those over-sized hard rubber balls you see at the gym. We have one at home, and this made a great, if bouncy, seat for one lucky guest.
4. Camping Chairs - One of these must be lurking in the back of the closet somewhere. Right?
5. Step Stools - We keep a two-step folding stool in the kitchen to reach the tops of cupboards. It's actually just the right height to pull up to the table!
6. Plastic Bins - We have a few of these in our closet holding old papers, decorations, and the like. Stacked on top of each other, they're as good as milk crates.
7. Bikes - This is when we knew we were truly desperate. And yet, that top tube on the bike makes a pretty good seat if you don't mind leaning!
Our guests were pretty good-humored about the situation, but we think we might just break down and invest in a few folding chairs for next time. These Nisse chairs from Ikea are stylish, fairly inexpensive, and would easily fit in the back of a closet until we need them.
What other alternative seating arrangements have you found when desperate?
Related: Kitchen Gallery: Made for Entertaining
(Image: Flickr member purplbutrfly licensed under Creative Commons)
Straw Mat from The ...

Does anyone remember the Thanksgiving dinners when the piano bench was put into service as extra seating? Cause I was little, I always had to sit on the piano bench with some relative!
we recently had a similar problem. don't forget the computer chair. ours is a slim, simple one from ikea so it fit in at the dinner table just fine. also, we had to pull another chair from our living room. it wasn't too bulky. it's not a big upholstered armchair or anything. but, it does lounge back a little and was tad too low for our table. but we sat in the weird chairs and offered the normal ones to our guests. so it worked.
How about some cushions on the floor around the coffee table?! That's my favorite spot to eat dinner!
Milk crates with cushions on top work great. And I always find that guests don't feel put out if they're asked to bring stuff--they _want_ to help!
And like ScottyT, I've had some sit-on-the-floor dinners too.
Oh, and you can also slide the table over by the sofa, and use that for seating on one side.
A friend of mine has a small place so we just sat around his coffee table for dinner. I bought some nice wooden folding chairs from Cost Plus a while back. I think they were two for $25 at the time and come in different finishes. Definitely worth the investment.
We have the Nissa chairs in yellow for extra guests. They're comfortable for long evenings at the dinner table and they look great on the chair pegs in our kitchen.
bring in the chairs on your patio or deck.
I have a folding table and chairs I keep just for such occasions. You can usually find folding chairs on craigslist inexpensively, and I know my set wasn't terribly expensive from Target to begin with.
i had the same situation and my guests didn't mind bringing their own chairs. i also used a large trunk to seat two.