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About the 200-page manual, you don't need to read all of them. Just pick the sections that apply to your needs. For example, you can read the Section 4 about sharing a folder, and read the section 6 about Group Account service. Most of these pages are screenshots only, so it does not take a long time to read. ( I still recommend reading it. )
>>> About how to make a document private.
By default, each user has his own private My Storage root folder. Files in your "My Storage" are private to you and other users cannot see them. So to make a document private, you really don't need to do anything as it is automatically private.
Now you can create multiple folders in your "My Storage", e.g. you can create two folders: Folder1 and Folder2 in your "My Storage". You can share Folder1 to your group or to other users. Other users can access Folder1, but they cannot see / access Folder2.
You can share different folders to different users with different permissions. Other people can only see the folder(s) you shared to them.
If you want to edit an existing share, or delete (stop) a share, you can go to Manage My Share page. From DriveHQ.com website, you just need to go to My Storage, then click on the Shares tab to edit the existing shares. From DriveHQ FileManager client software, you can click on Tools, then select Manage My Shares.