The best practice for User Authentication when working with SQL Server is Windows Authentication.
This will reduce your Administration overhead. So the best thing would be add these sql authenticated users to a Windows Group, say your AppUsersGroup that is currently accessing SQL Server.
1) Now open up Enterprise Manager, navigate to the Security Folder and under Logins choose new Login.
2) Now under the login name, choose Search, you can add members from the Active Directory
3) Now your users will be authenticated to SQL Server via Windows Authentication.
Hope this helps
Senior SQL Server DBA