Last time I checked, the exact rules were that all of the code had to be either typed in by the user or included by the app. So there's no real way to share code or collaborate other copy pasting into text windows.
I haven't used Pythonista myself, and it is true that, as I understand it, Pythonista doesn't directly provide the capability to use "pip" or pull code from GitHub. But it seems that these issues have been addressed via plugins for Pythonista. E.g.,
