TCArknight, you might want to put together some kind of readme or quick guide for folks to work from on how to use github and potentially git itself to work on this. Like maybe setting up branches for specific sections of the work or such. This guide for github might help (
https://guides.github.com/activities...-world/#branch). Maybe something like Git for WIndows (
https://git-for-windows.github.io/) or other of the Git tools (
https://git-scm.com/) would be helpful for some folks as well.
Then it's a question of coordinating your collaboration, which you could either do here or potentially on github to figure out who might be working on what as well as what needs to be worked on. I would think you might be on the right track for now with separate .user/.dat files for specific sections folks might be working on but it would probably be helpful for folks to know where things stand to get an idea on where to continue the work. Once it's ready for primetime presumably it would be best to consolidate it all back into a single .1st and .user file if it's going to be published through the standard update mechanism, anyway. Personally, as a user of HL, I find the one setting that is a tad annoying to work with is the older Hellfrost files because they aren't consolidated into a single file. That could just be me, though. Certainly for collaboration purposes I would guess breaking them out like that is probably a lot easier.
I really look forward to seeing how that works out. I may have to see how that works for you and maybe look at doing that for some newer setting files whenever I can get back to working on some. I know WWI needs to be done and The Goon and Fear Agent are on the horizon now too...