Depends on what you mean by a basic demo.
There are demos that show the game in early stages of development. These are generally large worlds that are sparsely filled and riddled with issues. These demos usually look like garbage to the untrained eye.
There are demos that are basically the entire completed game but with a small level. These often need to demonstrate one of every object and consequently require everything to be complete and polished.
To get a demo of a networked game you also need servers to be implemented and available, not just the game client. Exactly how much of that needs to be implemented depends on your design.