Though, when speaking about SQL Server logins, these users will might be orphaned as the corresponding SQL Server logins might not exist anymore. A similar case is when restoring a database on another SQL Server Instance.
In this case, you can use the following stored procedure for fixing such issues: sp_change_users_login
EXEC sp_change_users_login 'Auto_Fix', 'UserName', NULL, 'Password';
The above example will create a new SQL Server Login for the given user name and use as a password the given password string.
You can also assign existing logins to orhaned users.
For more information you can visit SQL Server Books Online.