-- Showing a report of the current user to login mappings
EXEC sp_change_users_login 'Report';
GO

-- Automatically mapping a user to a login, creating a new login if it is required
EXEC sp_change_users_login 'Auto_Fix', 'Mary', NULL, 'B3r12-3x$098f6';
GO

http://msdn.microsoft.com/en-us/library/ms174378.aspx

Maps an existing database user to a SQL Server login.

security

SQL