Do they have some sort of Minecraft version control system? Can people work on a local copy and then merge their changes into the main branch?
I've also been meaning to play psuedo-multiplayer with my friends by us each claiming a region in a seed. Each region being 32x32 chunks and saved to a separate file (.mca), you can just drop their latest region into your world folder to see what they've been up to, or play on LAN and send it back after.
EDIT:
After looking at the website a bit more, it appears that there is a sort of VCS that is managed by the admins. Users make changes and send them to the admins, and if they are OK then they get merged into the main world.