Collaboration Software / Setup

Hi,

I need to set up a collaboration system for a project that i will be overseeing soon.

There are 4 different companies, with 3 people from each company feeding into the project.

The material that we need to share, access and collaborate with, will purely be documents (word, excel, pdfs).

Can anyone recommend a system that has a very easy / small learning curve, as some of the users are not so advanced when it comes to IT.

I was thinking of using Google Drive or Dropbox, however Google Drive allows you to comment and intereact with the users?

Thanks in advance for any advice on this.