I decided that since my daughter now has a boyfriend, I'd invite him to live on the lot as a roommate. The whole family adores him so it just made sense. The way, we could closely monitor the relation...
You can use the add to household functions when shift-selecting a Sim which he mentions on the Xbox, you just have to enable testingcheats first (which is also true on PC).
To enable testingcheats on Xbox hold all 4 shoulder buttons at once to bring up the cheat console, and then type "testingcheats true" (without the quotation marks). After that, shift-select a sim and one of the options is to add or remove a sim from the currently active household.