If the server has been set up for integrated or mixed security, at the time the login security mode was set, the name of a default login ID (usually guest) could have been specified. Under integrated security, when an authorized user (one granted login permission by SQL Security Manager or xp_grantlogin) who does not have a SQL Server login ID logs in to SQL Server, the user is automatically mapped to the default login ID. If a default login has not been specified, then such a user is denied access to SQL Server.
If a default login was specified when the login security mode was set to integrated or mixed, then that login is added automatically by SQL Server.
A default login ID on SQL Server only allows a user to log in to the server. To allow a user access to a database (without adding a username), a guest user must be added to the database. For information, see About Database Users.