Excel 2003 and later has a feature called shared workbooks. This should let you do what you want. It does come with certain limitations, but for your use case, these shouldn't be a problem.
Specifically, you cannot:
- Create an Excel table
- Insert or delete blocks of cells (you can only insert entire rows/columns)
- Delete worksheets
- Merge or split cells
- Add or change conditional formats
- Add or change data validation
- Create or change charts or PivotChart reports
- Insert or change pictures or other objects
- Insert or change hyperlinks
- Use drawing tools
- Assign, change, or remove passwords
- Protect or unprotect worksheets or the workbook
- Create, change, or view scenarios
- Group or outline data
- Insert automatic subtotals
- Create data tables
- Create or change PivotTable reports
- Write, record, edit, view or assign macros
- Add or change Microsoft Excel 4 dialog sheets
- Change or delete array formulas
- Use data form to add new data
- Work with XML data, including:
- Import, refresh, and export XML data
- Add, rename, or delete XML maps
- Map cells to XML elements
- Use the XML Source task pane, XML toolbar, or XML commands on the Data menu
You can of course access all of these features again after you unshare the workbook.
Another option is to use Sharepoint's Excel services REST API.
Though I'd personally just use a proper database as that's what you're really using the spreadsheet as. You can always use an Access/Jet database and then export to Excel spreadsheets, or you can use a MySQL DB and connect to it using OpenOffice/LibreOffice Base and export the data to Calc and save as an ODS spreadsheet.