Regular Users
When you create a login on the control panel you choose a home folder for the user. Users are jailed to their home folder, they have no visibility outside their home folder, they can only see the files and sub-folders you intend them to see. They can not see files or folders belonging to other users, indeed they can not even know they are there.

Admin – The Special Login
Regular users created on the control panel are jailed to their home folder. However there is one preconfigured super-user: admin. Admin can roam the entire site and can download files from, and upload files to the regular user folders.

You generally reserve the admin login for your own use and make regular users for your customers, etc.

Sharing Files
If you want a workgroup to share files you can do that in two ways: 1) Give the same login to more than one person; or 2) Set the same home folder for multiple logins.

Creating a Hierarchies with Supervisor Logins
You can make a hierarchical structure in which one of your users has rights to the home folders of other users. For example create a login for the Sales Manager with home folder /sales, then create logins for the sales staff with home folders of /sales/joe and /sales/jane.

Since every user can see their home folder or below the Sales Manager can upload to and download from the sales staff folders. But the Sales Manager can not see any of the folders you made in another tree which you may have made, for example, for the marketing department.