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Yep, you set up each developers machine as a remote, but you pull from them (rather than push). Helps to pull to a separate branch though.

Setup internal dev box, push code there and from there to github...

Well, ideally you'd have DNS setup internally so you don't have to use raw IP addresses, but essentially yes, you map each of the people with which you're collaborating as remote repositories. Because they are remote repositories, all on their own. There is nothing special between your repository on your machine and the repository on github.