At the moment, you can share a Playlist from all Projects on your site with any Client Users you've created. We've heard feedback that it would be useful to lock specific Client Users to certain Projects, that way, if they're not allowed access to a Project then a Playlist on that Project cannot be shared with those Client Users. This could work similar to the 'See Assigned Projects Only' permission where users need to be linked to a Project to have access to that Project's data.
If you'd like to see this supported in the future, please click 'Me too'.