How to Render others models in Grabcad and upload to get Render Bender Badge ?

Great question, I just asked the GrabCAD staff this same questions yesterday.
I'm paraphrasing, but the general concept is:
1. It is broken.
2. They are looking into fixing / changing how it works.
3. The Render Bender badge may be going away.

I think they are open to ideas an suggestions on how you might like the system to operate in the future. Send them a message (big red question mark in the lower right of most pages).

For a while, I've been rendering and uploading a render to the comments section of the project page. It does not get you any closer to a badge, but it is still nice to render an otherwise blah model to get someone extra likes.

Now, just add your renderings in the comments and they'll be counted.