I need to convert an Excel spreadsheet and allow multiple users to update it over the web.

SpreadsheetConverter can only perform online calculation and collect form data, not show data that was previously saved as a file. It seems to me that you need some kind of online database. We have created such a solution:

The turn-key solution called SpreadsheetServer will let you view, edit and save spreadsheets using any browser. You can share a spreadsheet online and the users do not need Excel at all. All data will be stored in a relational database. The data can be filtered, consolidated and accessed through reports and as Excel spreadsheets. Spreadsheets or single worksheets can be protected by password.

It is intended for applications like custom-made time-reporting, expense reporting, task lists, calendars schedules etc.

You can read more at

http://www.spreadsheetserver.com

We are not selling SpreadsheetServer at this time but a new version will be released for sale later.