An online wall may be occupied by clicking the "Online" button. Only one person can occupy a wall at once. If you successfully occupy a wall, you can work on it, then hit save. The next person to occupy the wall will see any work you did and can in turn also work on the wall. Just like a real wall.
The first "Online Wall" has been enabled on the public testing branch for the factory environment. The interface can be found on the back wall where the huge windows are.
Right now this feature is experimental and may, or may not make it to the stable branch but my hopes are very high it will. The sooner this feature is put through it's paces the sooner I can extend it to the other environments so more walls are available for people to work on before finally releasing it.
I also recommend for now that you keep younger children away from this feature and the testing branch in general as users will no doubt create content unsuitable for younger players. Obviously a warning will be added before the feature is released.