Database backups from one MS SQL Server (server1) could be used to create a copy of the database on another MS SQL Server (server2). But existing user to login mappings are not restored correctly even if same logins exists on server2. The databases restored in this way could be made accessible again to these users by using the sp_addalias system stored procedure. This SQL statement should be executed after connecting to the restored database on server2 using ISQL/W or some other query tool.
sp_addalias ,