You will get boarded from the lounge. You can choose to get a redcap who will take you out, or you can choose to go with the group from the lounge. There is normally an escort who will walk you to the platform/train.
And unless something has changed since last January, as soon as you get settled into your Sleeper, you will be invited to Wine and Cheese in the Diner.
It should also be noted that they typically try to board the sleeper pax from the lounge around 45 minutes before the scheduled departure of the train. So you'll want to be in the lounge at least 1 hour before departure. Sometimes the conductor will even come to the lounge to collect the tickets, rather than doing it onboard.
This past Monday, I was told to return to the Met Lounge at 8:00PM. The sleeper passengers boarded shortly thereafter. In this instance, the conductor didn't collect the tickets in the lounge; the SCA did once onboard.