The roomette is indeed an upgrade from coach, since you have a private compartment with two seats in it. You can close the door and curtain for privacy if you want, and of course at night there are two beds to sleep in. Additionally, all meals are free when taken in the dining car.
Now that said, first, Amtrak does not have any provisions for sharing with a stranger. If you happen to meet one or can find one somehow, then you are welcome to share the room, Amtrak won't stop you. However, I would consider sharing the Viewliner Roomette to be quite awkward, since the toilet is not within a closet or curtain area. You'd be sitting/standing there in all of your glory, unless the stranger steps out into the hall. And there really is no place to stand in the hall.
The only other option is to walk back to the coaches and use the facilities there.
And if you do plan to sleep, then I would expect that a certain amount of trust would need to be established, since again you are both within the same little room.