I want to set up a single folder that will be accessible only to me (group administrator) and a single sub-user. 

Moreover, I want to restrict that sub-user to be able to see ONLY that one folder. 

How do I do this?

If you don't have a group account, please logon www.drivehq.com, click on My Account --> Group Account to upgrade to a group account. Group Account service is free in itself and offers 1 free sub-user. You can order more sub-user licenses at an extremely low price of only $6/user/year.

You can easily create folders in your account. You can create a folder for each sub-user, then share the folder to the sub-user. When a sub-user logs on, he / she can only see the folders shared to him/her in the special folder:

   \DriveHQShare\YOUR_USERNAME\SHARE_NAME\

You can share different folders to different users and set different levels of access rights.

You can also share the same folder multiple times to different users and set different access rights. (Note you need to use different Share Names )

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For more advanced users, you can even share folders to different groups of people and set different levels of access rights.

Yes. Logon www.drivehq.com, click on Group Admin, you can find a link "Group Options" and check "Group members cannot share folders".

Also, do not allocate storage and download bytes to the sub-users. Instead, you can create folders in your own account, then share different folders to different users with different levels of access rights. When your sub-users logon, they can see the folders shared to them in the special virtual folder:

\DriveHQ Share\YOUR_USERNAME\