If you're in a country where hotels don't ask for ID, the additional guest can just state the primary guest's name and the hotel would be none the wiser. As long as the name matches their apparent gender and in some cases ethnicity.... and they don't say "huh? What's Diamond" when the staff thanks them for being loyal.

Many hotels don't ask status holders for a card, even on non-prepaid stays. Of hotels that do, very few of them care about whether it is the same card that is on file.