Vista - What happened to User2?

My new eMachine is running Office Standard 2007 on Vista Home Basic.

This computer has four users on it. User1 is Administrator. Users 2, 3,
and 4 are Standard. I alone am User1, 2, and 3. User4, someone else, is not
involved in this problem.

User2 is my daily workhorse. User3 is reserved for a special project.

As User2 and Owner I want to "share" an Excel workbook with Users 1 and 3 as
Co-owners.

At the outset I did not understand file sharing. I thought I could just
email the file (=workbook?) as an attachment to User1 and User3. When I did
I found out they could not function as Co-owners. So I decided to start over.

Since at that point, after some use, User3 had the best version of the Excel
workbook, I deleted the inferior versions in User1 and 2.

Working from User3 I tried to add User1 and User2 as Co-owner and Owner
respectively. I successfully added User1 as Co-owner. When I tried to add
User2 I got the error message: "Cannot locate User2." Restarting the
computer yielded the same message, and so did restoring the inferior version
of the workbook to User2.

Where do I go from here? I'm hoping the solution will let me preserve at
least an inferior version of the workbook.