To move in you have to do loads of friendly interactions. Then in the box up at the top left where theres the relationship shown it says 'foreign sim' thinks that 'Your Sim' is being Friendly. Then click again for friendly then an option should appear to ask to move in, you might have to click more on their to get other options!
I don't have WA yet butthis is how you do it in Sims 3, getting Engaged then have Private Wedding is another route.