Portal : how to attach folders / documents to portal user ?

[k26]

I can create a portal user, folders, documents in the folder... but I cannot understand how a portal user can only see files in a directory for him specially.
I have played with portal roles, portals, entities, etc... and read the documentation but it seems it cannot be done, unless maybe I create 1 team for each portal user....
Can you please give me some links or tips to understand and make it work ? Thanks