As the title suggests, I can't figure out a way to share a folder outside my home folder using Ubuntu One. When browsing to my work partition using Nautilus, every folder has the "Share" option grayed out.
['https://askubuntu.com/questions/78924', 'https://askubuntu.com', 'https://askubuntu.com/users/39959/']
I just did a test, created a new folder */new* and gave it permissions 777. When right clicking on it, Ubuntu One options were greyed out. Also tried creating a shortcut in ~/ dir, but no luck either.

In short: No, you can't
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There are unsupported ways which will work. Think mount points under ~/

Using a bind mount as described above to a folder under ~/ you can sync any folder on your computer with Ubuntu One. 

I sync folders on a separate partition with Ubuntu One this way. 

Like I said, this is not really supported, but it does work