The two guests is the rule for when you're not upgraded. If you check the Gold terms, it doesn't mention how many get access to the lounge if a Gold member is allocated a room type that confers Executive lounge access.

Therefore, my understanding of the rule is, if the room is booked for the right amount of persons and the room is upgraded to a type that confers access then all persons get access to the lounge. I point out the correct number of persons, because extra guest stays free doesn't make 2 + 1 per the rules. It's just for one room.

At the Millennium Bangkok they deliberately limit occupancy to two for those staying in an Executive room, despite it being the same size as the Premium and Deluxe (standard) rooms that allow 3.