You can make a model collection public, or private.
This means that you can now add collaborators or restrict viewers for any collection you own. To do so, just click the "share" link on your collection's "details" page. Collaborators can add and remove items in your collection; they can also edit details such as its name and description. And if you make a collection "not-publicly-viewable", only people you choose can see it.
(via The Official SketchUp Blog)