In case it was of interest to anybody, I found that I'm only given the option to ask another Sim to move in when they perceive my Sim to be "sociable" and then it is located within the Friendly interaction context menu. On account of this being the only method I ever use, I'm unaware of whether Sims can be moved in by any other means, although I'd imagine marriage would be a viable option. I hope that's useful...?