Huddle up!

Huddle is a network of secure online workspaces where you and your team, your customers (even your friends) can come together to work on files, plan projects and collaborate on ideas. Each individual workspace contains files, notes, reminders and information on your project. You can even manage multiple ongoing projects across different teams with huddle.

huddle-up

I came across huddle when looking for a collaboration tool. I found it a very decent solution for groups with a flat structure. You can do the most basic file sharing things at a reasonable price (freemium model, with a fair enterprise pricing model).