Originally posted by Beckelhimer:

Yes, you can set up your SP world to be a server for others to join. It is more like Minecraft as in you have to be playing for others to have access to your game world to play in. The world is still YOUR world so they won't be able to play in that world without you. But yes, you can open up any world you have as a server for others to join with or without a password.

This isn't true either. The game comes with a dedicated server executable that you can run to host your world at all times. Simple to get running on a LAN, slightly more complicated for open Internet access. Or you can hire a hosting service and upload your single-player world for them to host. That's got its own difficulties, but none of them are insurmountable.