I got this from them in September 2020 (booked for 3 adults on points using the 2 existing beds in room, and no mention of charges when booking):

"As redemption stay is only covered for two adults, it will cost JPY 7,500 (exclusive of tax and service charge) for 3rd guest/per night. Please be assured that it includes the access to Executive Lounge for 3rd guests"

Stay ended up being cancelled, so no further experience here.