Okay, this is very alpha but also insanely cool.
VisualInk now supports live collaborative script writing sessions! Whether you just want to play on the demo editor page, or you've signed up and you want to collaborate on a script owned by your account, just hit the "Copy live edit invite" button at the bottom of the screen and send the invite to a friend.
Warning: the session only persists as long as you stay on the page at the moment and doesn't warn you if it breaks. If you run your script, browse away, reload the page, etc, you'll need to send out new links. We're working on a proper release!