Originally Posted by
Infinite1K
Mori tower lobby has some entrances/exits into the mall. If you can get to the Starbucks West Walk/2F (which is actually the ground level as the outside area with the spider, etc.) look for a corridor between Balenciaga and the boutique MUSE de Deuxieme Classe Roppongi Shop. At the end there will be an escalator. At the bottom, make a left and you’ll enter the covered motor lobby of the Grand Hyatt and you can then make another right into the hotel.
I think OP wanted to go in the other direction, but I'll just add that if heading to the hotel, I believe there is GH signage to get into that corridor, and there are definitely signs at the bottom of the escalator. That's actually the path that I took to get to the hotel from the Roppongi station (after entering the mall from the courtyard area to the north) before my first stay there without really knowing where the hotel entrance was precisely.