To work collaboratively, a new tool could be really interesting: SageMath Cloud
Collaborative edition of LaTeX, Sage, IPython NB, Terminal, …
Even in the chat markdown and latex syntax can be used. Via terminal it is easy to git pull/push, and probably possible to use dropbox or other syncing tools.
The project is still quite new, and there is no “premium account” policy like sharelatex or other sites of collaborative work. For the moment it is funded partially by Google and the US.
See for yourself: https://cloud.sagemath.com/