How do I share a folder with a friend with Ubuntu One?

According to the features page:

Share folders with contacts

Share folders with your trusted colleagues.

Folder sharing is integrated into the Ubuntu desktop. Right-click on any synchronizing folder and pick contacts from your Evolution address book to share with. You can even grant the recipient read-only access for simple file viewing or write-access for complete control. Share recipients must have an Ubuntu One account to receive shared folders.

And for something with pictures to show you how to share a folder with someone using Ubuntu One, check out: https://one.ubuntu.com/help/tutorial/install-and-setup-file-sync/

You need to log in to the UbuntuOne website.

Click on Files to see the list of files and folders.

To the far right of the folder you want to share, click on More, and then Share folder.