Create a workbook for multiple users on different computers?

I am trying to create a workbook for my sisters photo business. Its taking off but they need some organizational help. My workbook would help them (My sister, he husband, and my mother) but the problem is that they all need access to view and update it. They are not organized so sending e-mail attachments back and fourth will probably not work. How can I get to where they can all use the same program from thier PC's so that they can all be on the same page?

I have looked into Zoho or Googledocs, they seem like good posibilities. Does anyone know much about them? Any other ideas? Thanks.