No one owns Demon's Souls yet. No one. The game doesn't release for another few days. Procurring an early copy is impossible. Even if you're the President.
But, hypothetically, if you've somehow managed to weasel your way into a copy of Demon's Souls before its release date, Atlus would like you to know that the game's online servers won't be live until October 6. The publisher told us as much in a press email.
For now, you'll just have to make do with single player. Or you would, if you had a copy of Demon's Souls. Which you don't.