The Transporter is is peer to peer, so it is private – your data is never stored online and is encrypted during transit – so privacy is never an issue.
The data is only shared with others who receive your invite to do so – whether they have a Transporter or not.
Any files that are added to the folder will automatically appear on the Transporter devices and/or computers of people you have invited. When you create a top-level folder you will also have the option of specifying whether or not you want the files to be hidden and encrypted on shared devices.
If you are looking for something like Dropbox or Google Drive but prefer to be in complete control over your data, the Transporter might be for you. They come with 1TB or 2TB drives (or without a hard drive).