We tried to approach collaboration in a very simple way that doesn't require accounts and heavy realtime Google Docs-style co-editing. Our collaboration relies on versions and links. Every refresh creates a new version, so as soon as you pass a link to someone, they create a new version automatically. This way you're never editing the same version, even though you're editing the same "document". So it definitely is based on passing links around but making sure there are no conflicts. We're definitely looking into simple ways to notify you that there's a new version created by someone else so it runs even smoother.
You can think of it a bit like a tree system, where you start on the stem and as you share it with others you build independent branches. Hope it makes sense, we're definitely going to work more on that part in the future.