Re: Allow multiple users simultaneously
You're essentially trying to make a single user object do something it's not designed to do.
To make a multi user scheme really work, you would have to update each change made by the user as it is made, and not when the spreadsheet is closed.
You would also have to implement some form of locking scheme which locks the sheet at a row or column (or even cell) level which could be done using a shared random access file but would slow things down and you would still have the same problem as outlined above.
All in all, you need an application that supports multi-users and record locking at a native level.
As the other posters have said use a proper database (and that doesn't include access if you've more than 10).