If you are planning to invite someone in advance, call Amtrak and arrange for an "Open Sleeper" ticket for that person. It doesn't cost any more, as already mentioned, the cost is for the room not per person, and the other occupant only pays the rail ("coach") fare. That puts the person on the sleeper manifest, entitling them to meals, etc, that are part of the sleeper passenger amenities.
If you are getting lucky on board, well, that is a whole different matter...