Do you add water to the soup when unexpected guests arrive for lunch ?

Depends on the type of soup, the guests, and what else you have.

You could - add a can of vegetables and some chicken or vegetable stock, serve a little less to each person and sprinkle cheese on top and put a piece of nice bread on the side or some bread sticks, and/or add croutons to the top just before serving.

Frankly if they are an unexpected guest and there just isn't enough to go around I would offer them a sandwich (unless it's your mother in law).

No, unless it is condensed soup and needs to have water added (some soups are). If there is not enough soup, you should offer them something else. Bringing univitied guests is bad manners.

omg ill make sure never to stay for soup at someones house after reading this.