I need for someone else to add things to videos that I upload to YouTube. How can I share a private YouTube video with someone so they can edit and save it?
You can't share a private video with anyone who doesn't have access to your account. Someone mentioned posting it as 'unlisted' but that isn't private since anyone with the URL can see it. If you trust the person to not share the URL then post the video as unlisted.
Since the video is already completed it can't be "edited" in the true sense of the term, but rather add additional features YouTube provides (annotations, changes to title, etc.
You can connect your YouTube channel to Google+ page and then you can add managers.
In this way you can share private o non listed videos with your collaborators.
I believe just setting the video as "unlisted" works.
Unlisted means that the video is not listed on the site, but available for view for anyone with the link !