See the Excel HELP topic "Share a Workbook" which gives these instructions
plus some more information:
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Share a workbook
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Create a workbook you want to make available for multiuser editing, and
enter any data you want to provide.
If you want to include any of the following features, add them now: merged
cells, conditional formats, data validation, charts, pictures, objects
including drawing objects, hyperlinks, scenarios, outlines, subtotals, data
tables, PivotTable reports, workbook and worksheet protection, and macros.
You can't make changes to these features after you share the workbook.
On the Tools menu, click Share Workbook, and then click the Editing tab.
Select the Allow changes by more than one user at the same time check box,
and then click OK.
When prompted, save the workbook.
On the File menu, click Save As, and then save the workbook on a network
location accessible to the intended users. Use a shared network folder, not a
Web server.
Check any links to other workbooks or documents, and fix any that are broken.
