Selected Answer
What you are talking about isn't linking. It's sharing, and it means that several people have access to the same workbook.
From this base you can branch into several possiblities. One is that you want to restrict access of some parts of the workbook to some others while others are open to all. You can solve that problem by hiding some parts or use password protection. But bear in mind that such measures are intended to protect against accidental access and will not keep out determined hackers.
Microsoft has done a lot of work on File Sharing, on the server, on the Net and on the cloud. However, the subject is really rather remote from "Teach Excel".