How to use two different computers to edit same GitHub repository

I solved this problem using Dropbox, although which might potentially bring version confusions if used in a wrong way, according to our lab tech.

Anyway.
For Mac computer, download and use GitX
For windows, Github has its own interface if you want to try.
      just click the plus sign and choose add repository and browse and select the folder you want to add. The Git files should be synced by Dropbox so its history should show up.