In OpenERP for the movement you can control the visibility of directory based on the groups instead of controlling the operation like edit, delete, read on directory or files.

You can try to assign a group on the security tab on directory, by default all can see that directory and files of that directory, but once you put groups on it, only user belong to those groups can see that directory and files.

There is no further permission related to directory / files that can specify the read, write, delete operation on file or directory.

You'll need to use combination of 'Security Groups' of the specified folder and Attachment, ir.attachment, and document.directory.content Access Rights. I guess these will suffice to control access to documents.

Set the desired group on a the specified folder, then go to the that group's access rights and modify the ir.attachment Read, Write, Create, Delete rights per your desire. This will control access to documents through the Attachment button on top of the different forms. Add or edit rule for document.directory.content per your desire.