Invisible folder permissions. (#75)

en/sharing_collaboration/folder_permission.md

@@ -1,6 +1,6 @@
 # Use Folder Permission for Fine-grained Access Control
 
-For shared libraries or folders, you can set permission to its sub-folders for a person or a group. Available permissions are the same as the ones described in [sharing libraries](./sharing_files_and_folders.md). Here are some use cases:
+For shared libraries or folders, you can set permission to its sub-folders for a person or a group. Available permissions are the same as the ones described in [sharing libraries](./sharing_files_and_folders.md). A special "invisible" permission is added in version 11 Pro Edition. Here are some use cases:
 
 * You shared a library to a group with "read only" permission. You also want to allow a specific member of the group to modify files in one sub-folder. You can grant write permission of this sub-folder to this person.
 * You shared a library to 3 groups. There are 3 sub-folders under that library, one for each group to upload files. However you don't want users to change files not belong to his/her group. You can grant write permission of a sub-folder only to its group.
@@ -33,3 +33,22 @@ If a "read only" folder is synced to PC with the Seafile desktop App, changes un
 Here is a more complex example:
 
 Supposed the folder structure looks like "A/B", user "John" is in group "Staff". If you set "read only" permission on folder A for John, and, you also set "read write" permission on folder "A/B" for group "Staff". Then John's permission to folder "A/B" is still only "read only". That's because, according to rule 2, "read only" permission is first matched for folder "A/B" and user "John", which is inherited from its parent "A".
+
+## Invisible Permissions for Sub-folders
+
+Sometimes you may want to share a folder to a group of users, with some sub-folders only visible or invisible for specific users. In version 11 Pro Edition, we added "invisible" sub-folder permission to support this use case.
+
+To set invisible sub-folder permissions, just open the folder permission dialog as described this previous sections. You can then choose "Invisible" permission from the drop-down menu.
+
+![](./imgs/folder_perm_invisible.png)
+
+Suppose that you share a folder to a group of users, which includes user James. The folder you share contains 3 sub-folders: A, B and C. Let's go through two typical use cases for invisible folder permissions.
+
+In the first case, you only want James to see folder B, the other users in the group should not see folder B. You can simply setup the folder permission like the following:
+
+1. Set sub-folder B invisible for the group.
+2. Set sub-folder B read-write for James.
+
+This way the permission for James will overwrite the permission set to the group, making B only visible for James.
+
+In the second case, you don't want James to see folder B. The solution is simple: you just set sub-folder invisible for James.