If you can store the file on a common folder that everyone can access, you could
use Tools|Share workbook (xl2003 menus). Be aware that there are lots of
features that are unavailable in a shared workbook.
And lots of people around here recommend that you stay away from sharing
workbooks. Files get corrupted is one common complaint.
You may want to look at a different application that's built to allow
simultaneous updates -- like a database program (Access???).