Hi Joe

You are quite correct about shared workbooks and Pivot Tables.
You can link another workbook to the Shared workbook, by telling the PT
to Get Data from an external source (albeit it is another Excel
workbook). It will not refresh whilst the workbook is shared, but once
you remove sharing, it can be refreshed, then set the source back to
being shared again. Probably not the best way of doing what you want.

Before resorting to VBA, if it is only a count of the items belonging to
each Order numbers that you want, then you could do the following.

Select Columns A and B of Sheet Summary
Data>Consolidate>use dropdown to Select Count>tick Use labels in left
column>Reference>click Browse and select columns A and B of Detail>OK

I am assuming that column A of Detail contains Order Number and column B
will also have values contained on each row.
Adjust as appropriate.

Just tried this in a Shared workbook and it works fine.