You will need to know her booking number to book anything on her booking.
I have booked my sister’s excursion for her under her booking # for the same excursion I booked for myself, I had her credit card #.
You will receive your excursion voucher in your cabin. It will tell you time and place to meet. Meet together and the staff with give you the same colored number sticker for that excursion. If more than one bus doing the same excursion, it will be the same colour but a different number.