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...
To make them a roommate you have to ask them and they have to accept which I think is guaranteed if they don't currently live on a different lot so that part would be easy enough I think.
I just wasn't sure what you meant by adding and removing them simultaneously to the HH.
Also the biggest problem with this idea is limited HH space with all of these kids running around.