Last year we added three people at check-in to the Hotel Cheyenne and the additional people got free breakfast.
I presume if we book half board for one person, then add the AP holders at check-in, that the additional people won't get free half board?
You're presuming right. When you add other people to the room they will get free breakfast at the hotel but no half board, no park tickets nor any other options you might have booked.
Remember to inform the hotel a few days upfront that you will add more people upon check-in or you might very well end up with a room with just one King size bed instead of the usual two Queen size beds.
Thought so, thanks!