Haven't done it too recent but it hasn't really changed as far as I'm aware.
The seats are ex HST First Class seats, so are comfortable enough. There is also a luggage/bike section too but unsure if you need a reservation for bikes, though I can't imagine you would need too.
The last time I did the seated part I only got bit of sleep as there was a group of business men who were intent of staying awake all night talking loudly about this that and the other, but other times it can be quiet, just depends really.
I can't help you with the Groupsave question, but you can walk onto the seated sleeper with normal return tickets on a walk up fare, so I would imagine Groupsave would be valid, could someone confirm?
Cheers,
James.