Not "massive" at all. But it's not a demo anymore. I haven't looked under the hood in a while since back when it first came out but it used to just create NPC's mirroring your other players actions. That was a long time ago though, I can't imagine it working this well still using that method

Edit: Here is the list of features, looks like it's up to 8 people now. But I think it probably does still use that old method, based on the way this is all described