When unexpected guest arrive you don't have to wait until they leave to eat.
Just improvise ! Add food items that cost the least to purchase and the easiest to add without making your guest feel they are an inconvenience.
Many food items fits the bill.
Potatoes, Long grain white or brown rice, barley,pasta, lettuce and many other .
Potatoes add 1 per additional person eating. Microwave: serve mashed potatoes instead of baked. Rice can be served as a starch by itself or added to increase volume. (ex: add to soup, burritos, casseroles as well as barley and pasta.
If serving a salad increase the lettuce and serve the salad into individual salad bowls.
And as far as dessert easy!! Always have a large jar of applesauce on hand, or a couple of boxes of vanilla and chocolate instant pudding, graham crackers or nil la wafers.
Pour applesauce into small individual serving dishes, sprinkle with cinnamon, stand 2 small graham crackers to one side like rabbit ears and serve.
Or make pudding according to directions, pour pudding into small individual serving dishes, layering each flavor, put a dollop of whip topping in the middle and serve.