Excel has a Sharing feature that allows multiple users to update the file at the
same time. (In xl2003 menus, it's under tools|share workbook.)
But most people would not suggest using this. Lots of people have had trouble
with this -- and there are lots of features that are disabled when you share a
workbook.
If this is what you mean, you may want to consider using a real database
application -- maybe Access???? These programs are made for this simultaneous
updates.
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If you meant that you wanted to just provide a workbook that others could update
(not at the same time), you could save the file with a password.
Show the SaveAs dialog.
Tools|General Options|Give the file a password to modify.
(make it memorable!)
Then you can share that password with your updaters and not with the readonly
crowd. (Ask that the password not be shared with others!!!)
Like any other password, there are sites that help(?) crack passwords.
Another option would be to put the file on a common network share that only some
people have read/write/create/delete access -- and others only have read
ability.
You may need your IT staff's help for this.