It is not Multiplayer. Its a single player game set in a basic mp world were you can see the other people that would enter into your lobby if you were in the area as them. You can not trade or co op. I believe you can kill them and that's it. But to see another person is like trying to win the lottery. They did say after sales and games released and the interest in the MP side go's up they may look at doing more for that side.
Thats the same thing I've seen and read from various interviews with Sean. Something similar would be Journey, where you have a chance to come across other players. NMS will have a similar thing, execpt there will be other players you can meet, but the distance between each other will be vast to the point you may never see another player even if you play the game for however long you'd play it. You have a slightly better chance of coming across other player's discovery trail.
Its been said that if you come across another player, you can't interact with them in any way except seeing each other (unsure about weapons fire). If you drop something, they can't see it or interact with it.