You should be able to share the folder with your collaborator - it's easiest to create a folder for them (to keep all your other Public stuff, well, Private), then click on the row the folder appears in on the menu. This should highlight the line and a Share option will appear on the far right. Click on that, and you'll get a dialog box allowing you to send an email to your collaborator, inviting them to that folder.
Hope that helps!
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