You can create a user Group and allocate just one user to it if you want to. The way permissions work in User Groups is a Joomla facility, jDownloads just complies
You can set all Downloads to allow Public Downloads by setting what are sometimes called the Component Permissions. It may alreasdy be set but to check on the jD Control Panel click on the Options tab in the toolbar and set Download permission to Allow for the Public User Group.
Remember that Permissions allow things to be done, Access allows thing to be seen.Colin