Use users / logins from another database
This means that database users have no matching server logins.

That is, each database sys.database_principals has no match in sys.server_principals

For Windows logins this is easy. This generates your missing CREATE LOGINS

USE MyDB
SELECT
    'CREATE LOGIN ' + QUOTENAME(SUSER_SNAME(sid)) + ' FROM WINDOWS'
FROM
    sys.database_principals DP
WHERE
    DP.type IN ('G', 'U')
    AND 
    NOT EXISTS (SELECT * FROM sys.server_principals SP WHERE SP.sid = DP.sid);


For SQL Logins, you need to recreate it with the correct SID.

SELECT 
    'CREATE LOGIN ' + QUOTENAME(DP.name) + 
                ' WITH PASSWORD = ''ChangeMe'', SID = ' + 
                CONVERT(varchar(128), sid, 1)
FROM
    sys.database_principals DP
WHERE
    DP.type = 'S' AND DP.principal_id &gt; 4 
    AND 
    NOT EXISTS (SELECT * FROM sys.server_principals SP WHERE SP.sid = DP.sid);


Note that sp_change_users_login is deprecated.

Now, if your SQL Logins already exist, then the names match but the sid values are different. For this you use ALTER USER use the LOGIN option.

Finally, the SQL Login passwords can be recreated if you have a backup of the "old" master database. If you restore this as, say, FixLogins then you can use this

SELECT 
    'CREATE LOGIN ' + QUOTENAME(DP.name) + 
                ' WITH PASSWORD = ' + CONVERT(varchar(256), SL.password_hash, 1) + ' HASHED, SID = ' + 
                CONVERT(varchar(128), DP.sid, 1)

FROM
    sys.database_principals DP
    JOIN
    FixLogins.sys.sql_logins SL ON DP.sid = SL.sid  
WHERE
    DP.type = 'S' AND DP.principal_id &gt; 4 
    AND 
    NOT EXISTS (SELECT * FROM sys.server_principals SP WHERE SP.sid = DP.sid);