There are indeed others collaborating using Scrivener, me for one. While I can see that anyone using GitHub in the normal course of their work-life would be tempted to try to use it in relation to a Scrivener project, my feeling is that, given the structure of a Scrivener project it will introduce more problems than it will solve unless all the people involved are using Scrivener itself. The scenario you present
assumes that the edits will be done using some text editor, in which case they will not be integrated in the other associated files that form the .scriv, project, which will at a minimum cause the index file to become out of sync. Modifying internal files with other editing software is strongly cautioned against as the project can easily become corrupted.
And if all of you are using Scrivener for this purpose, from what you have said, it introduces other problems, including the .rtf format, as in your original post. Using “External folder sync” is one solution. The other, which I suspect most of us who work collaboratively use, is to put the active Scriv project in a shared Dropbox or Cubby folder. The key to success in that method is negotiating times for access to the project.
You might like to look at:
Mr X