(See the best response by Gooseheaded.)
As the title says, I'm looking to make a game with more then one person. Just collaborating with a one friend.

Anyone got advice on what file sharing stuff we should use to work on a BYOND game together?

I've done this in the past by just having a separate folder labeled "so and so's work" that I merge into a main copy of a game. Probably wasn't the best method, but it sorta worked.

Anyway, I'm open to any suggestions and thank you in advance.
Best response
You should give Git/Mercurial and GitHub a try.

It's a little daunting at first, but after you use it for a few days, it's actually super cool! I used Mercurial for many years in college, and nowadays I use Git for personal projects as well as at work.

I even use Git when I'm working on a project by myself!
Git is a good place to start for sure. IDEALLY each person would be working on their own personal fork of the code then merging things in using push requests as needed.

But that's not required, you can have multiple people working on the same fork/branch using the same merge functionality, you just lose out on a few organizational aspects.
Thanks guys. I'll look into Github =)