You can control who sees which file:

1) When you add a file from your Project Files screen you are asked to choose which people involved in the project can see it.

You can change who can see the file at any time by clicking on the Access section next to each file in the files screen.

2) If you add a file from a task then the person assigned and anyone you’ve nominated to watch that task can see the file.

3) All files are listed in the Library screen. However people will only see files listed there that they have been given access to.

Read more on Dooster and Your Files here