I was browsing the Marriott Marquis DC website today, and it lists 3 ways to access the M-Club; via meeting planner, Platinum or higher elite status, or through an upgraded room reservation (2 adults per hotel key).

This is the first explicit mention I've seen anywhere of a limitation per hotel key. I assume that if I reserve an M-Club lounge access room for 3 adults and am issued 2 hotel keys, then all 3 adults (and up to 4 adults) will have lounge access.