You can convert your workbook to a shared workbook. For complete instructions,
see 'Share a workbook' in Excel's Help. However, there are many limitations to
shared workbooks. Once you make a workbook shared, you can't change any of the
following: 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.

I can't answer your second question  because I don't know anything about your
workbook: what its purpose is, what kind of data is in it, etc.