I went to the co-op tab in the Map screen and it did not look like I had authorized my game to be open to co-op. Whlle playing, someone regardless came into my game and immediately transported me. I killed the game but haven't figured out why this happened. Is there a way to stay "online" (so the saves go to the cloud) and not get bothered by unwanted "co-op" players (or griefers/hackers, I don't know).