Restore from SQL Server 2008 to 2014
When you move a database from one server to another via backup /restore, the users within the database stay intact .However, the logins associated with the users might not exist in the destination server. 

For this to work as expected, you will need to move the associated logins and then check for any orphan users.


Use this link to get the script for transferring the logins. You can verify the logins with the users in the database and move only those:

How to transfer logins and passwords between instances of SQL Server (Microsoft Support)
Once the logins associated with the particular databases are moved, check for any orphan users. This blog will help you understand more about this: 

Understanding and dealing with orphaned users in a SQL Server database by  Atif Shehzad



EXEC sp_change_users_login 'Report'

EXEC sp_change_users_login 'Auto_Fix', 'user'