Roger,

You'll certainly be able to join them in the regular Sightseer lounge car, if coach isn't sold out you might even be able to sit next to them for a while also. Regarding meals in the dining car, when the LSA comes around to make your lunch and dinner reservations, just tell him that you have 3 in the party one of whom is in coach. It shouldn't pose a problem unless you get a bad crew.

It's hard to say if you'll get a sleeping car attendant that would let the third person come up to the sleeper for a bit, some will allow it, other's won't. However, you'll probably find that it's not all that comfortable to seat three in the room anyhow and won't want to do that for very long.

The Parlour car, if there is one, should be off limits to the third person but who knows what will happen if you try. You may however be able to buy the third person a ticket for the wine tasting, which would get them into the Parlour car for a bit. I've heard that when all the tickets aren't sold in the sleepers, that the crew will open up the wine tasting to coach pax willing to part with the 5 bucks. So again if you ask nicely, odds are that you could get three tickets.