Additional guests can be listed in a hotel reservation and be personally checked-in by hotels. And the hotels can do that even in the absence of the primary guest on the reservation or before the primary guest named on the reservation shows up. But since the membership benefits will generally track from whatever rewards number is already in the reservation, the Diamond as an additional/secondary guest checking in will be at the whims of the hotel/hotel staff on whether or not they want to provide the breakfast vouchers and other at-hotel status benefits or not when it’s the additional/secondary guest who is the Diamond member checking in.