Can you play animal crossing without log in your password and user name?

Up to Four People can live at anyone given time in a Town (I think even Animal Forest the N64 version applies to this). You must play on one of the current people or make a person in order to play this is unless you are coming from another Town in which case there will be at least one resident of the town and at least one visitor.

In short Yes you need to play as a person, No you don't have user names and passwords.