Guest is a database user. It is not a login. Only Login names have passwords. Once you have logged into SQL Server using either your NT login or a SQL Server login (using a password) then SQL checks to see if you have a corresponding user in the database you are trying to connect to (SQL also checks if you are a member of a server role that will grant you access) SQL Server will also check to see whether a guest username exists. If so, the login ID is granted access to the database as guest. If the guest account does not exist, SQL Server denies access to the database.
The user name is used internally by SQL to check your permissions, You don't use this name to actually log in, so no password is needed.