@madameLee, hmmm...that's strange. I saw it before the EP, I swear. Or at least I thought I did...I feel like I gave one out the day before installing the EP. That and it just doesn't feel like an option they would tie into the actual EP.
It does only show up once you are good friends with the other Sim, and is called "Give Residence Keys" in the social menu at the root level (not under any categories), it has a picture of a key next to the words. If using the sims 4 normal social menus, it is usually on the first set of options that comes up, but if playing with the setting of "traditional social menu" it always is under "more choices" for me (and even then, only if the sims are good friends).
Oops, forgot to add, doesn't show up for children as far as I can tell, just teens and higher. I guess it would be creepy to give random kids a key to your place.