Actually selective sync doesn't fully handle this case. With selective sync you could choose not to sync down certain files to the computer, but then they aren't accessible at all. With ZumoDrive, the files are accessible but "deflated", meaning they take up no space but will stream and cache on the computer when accessed.
Zumodrive actually supports sharing either by a public link or to a private email address. There is a share option in the right click context menu as well as on the web file browser.