From the GitHub blog:

Writing code with other developers can be a great way to onboard teammates, get to know how your peers think, and learn new skills. Unfortunately, writing code together can be difficult to coordinate.

Now social coding is easier than ever with Teletype for Atom—a new way to dive right into code with remote collaborators. Work together in real time with your own configurations in your own programming environment on any file you can open in Atom.

I hear BBEdit is going to get this any day now.