We don't have support for multiple people working on the same code yet (at least with the Mac app).

But the benefit of editing locally and committing to Github is that that's not typically work flow - with Nitrous.IO for Mac - you can edit code, run tests (or refresh a page), make more edits, run tests again, repeat, and when you're done, then commit to a remote git repo (all with your dev environment being in the cloud, making it accessible from anywhere)