There is no guest login. If you don't have either a username and password or a windows login granted access to the DB, you cannot log in to SQL.
There are guest users in databases, that's for when someone has a valid login but no user mapped to that DB. Typically the guest user has no rights.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability