What you need to do is to assign each and every subfolder to it's own section. Then you need to assign a role to the user, that specifies which section(s) he is allowed to read.
But that's what I don't want to do cause at least I need about 100 differents users. That's why I need to group them in a folder wich belong to one section to keep the admin clear.
And for customer 1 you have given them permissions to access their folder, using the subtree, and the main Extranet folder.
If you want to move the user to their area look at this document, for redirecting after login. This tip can be used in your custom login.tpl page (overriding the standard tpl).