Want to find tutorial on controlling access to data

I am new to programming and am looking for tutorials on controlling access to data based on user logins for an app developed with Visual Web Developer 2008, ASP.NET 3.5.
Example, User A works for ACME 1, User B works for ACME 2. Each user should be able to add, update, delete information related to their companies records, but should not have any access to any other companies records.
Any links to references, tutorials etc would be greatly appreciated!!
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