When you upload a model, it gets dropped into a scene that can be viewed collaboratively by your team or your peers. Try visiting a project at the same time as someone else, and you'll see how that works. You can also click the Google+ button to launch into a video conference with your collaborative 3D space at the center.

On the editor: it's not a geometry editor, but what we do provide you with is the ability to create scenes/levels, and fine tune your materials. The power of the editor comes with the runtime that we'll be debuting at GDC this year. You can use our platform to host and manage your assets, create your levels, then pull those into your application and use Three.js to add dynamism to your applications. You'll see the first reference apps on that platform in the next couple of weeks, and a public release in March.

It's odd because the screenshots on the site do indeed show various editing controls (and make references to collaboration), but I also only get a viewer when I try it (even if I sign in).

Maybe the editor version hasn't been released yet?