Be aware that some hotels are only 21 and over and they technically could make a problem
though they have never stopped me B4
Firstly, add the guest that's checking into the hotel as an additional guest in the special request section while booking.
As per a conversation with SPG yesterday:
On a prepaid (cash) room, the hotel requires the guest that payed for the room to be present while checking in. If the room is booked with points, then anyone can check in, provided they are on the reservation as an additional guest (regardless of age).