I'm sending holiday greeting cards for the first time to friends, among whom some live with their partners (unmarried). I am not friends with their partners but consider myself on good terms with them, i.e. we are friendly acquaintances or friends by association. There are also some friends who live with their parents and siblings, where my relationship with them does not extend beyond a simple greeting and idle chit-chat when we see each other.
There are sources, like this one, that explain HOW to address groups of people, but I'd like to know the etiquette on when one should do so versus just address the friend.
For couples, I'm inclined to just address both people, as leaving one person out seems rude. But given that there is no direct relationship between my friends' partners and me, I was not sure if it would be awkward to address them. For families, I'm inclined to just address the friend (unless there's a relationship with the entire family).
This is in the United States, and cards are sent on my own behalf (i.e. not a family's or a partner's behalf).