Hi Greenwize, welcome to the forum.

If you use Excel you are going to lose control of the spreadsheet and how it gets used/modified. I've worked with several vendors that think they have a well-protected workbook or add-in, and they're shocked when they receive an e-mail with an unlocked version of their workbook showing all of their code, hidden sheets and formulas. If you already have a website, I suggest paying the designer to create web forms and scripts that perform the work, and store the information on your web server (or a linked data server) with access controls. There are free tools and free db software out there; no need to go purchase thousands of dollars of design tools and database licenses.

Linking a user-downloaded workbook back to your website and being able to write data would be risky at best. Unfortunately I'm not familiar with 3rd-party sharing tools as I don't use them in my area. Obviously any such tool would need to have security to prevent company A from viewing company B's information (amongst other measures).