Secondary options also mentioned included pbWiki and a forum (specifically bbPress).
This lead us to “Are there any good open source collaborative apps?” Options listed here included OneSky (Amanda Wong), TranslationCloud (Matt Bramowicz), crowdin (Iryna Bilyk, Marco Cevoli), Transifex (Yiannis Varelas), Pootle (Yiannis Varelas, Martin Wun, Ulta O’Broin), TranslateWiki (Evan Prodromou), otrance (Marco Steinhauser), mygengo (Ray Solnik).
Dave more recently posted another question, “What tools are available to create a book in a collaborative and distributed manner?” Stating the question in this same seemed like it would help generate the best ideas as it de-emphasizes the concept of translation (which, while accurate, is somewhat different in the instance of Scripture, as unlike most other documents being translated into English (or any other language), it has already been done so many times) and that it would also emphasize the scope of the project (as opposed to more general document collaboration).
The topic of using GitHub for collaborative authoring is not new, here is some of the more interesting articles/discussions we’ve found revolving around the topic:
Forks are different from clones as it “retains a connection to its originating repository.”
The author of a forked project can ask for its changes to be “pulled” back into the main codebase and Github shows in detail what changes have been made and would be merged.