Do you remember that du and Ni story? Well, Ni comes from ni, which is a way of addressing many people (such as you guys or you all). While Ni is not used anymore, ni still is (even though it isn't used very often, as Swedes don't often get the occasion of talking to many people at once..). So you use du when you are speaking to one person, and you use ni when you are speaking to more than one.
How would you say: