How to create apartment?

I'm confused by what you mean by "adding neighbors."

You don't add neighbors to an apartment. Story Progression generates them automatically to any doors marked for NPC use. To be able to mark a door, it has to be attached to a room that has the hidden room marker in it, and the area outside of it enclosed by a fence or hallway/vestibule/etc. that contains a public marker. Once the door is marked, over time, NPCs move in as the bouncers, mixologists, mailman and other service sims in your town.

Thanks for your response! I ended up posting on the EA website, and PuzzlezAddict helped me out!

"You are a genius! So just to reiterate for any other players having this issue, after I downloaded Buzzler's Fix NPC Door mod at Mod The Sims and added my playable family, I set my NPC door and moved Olive Kipling in through MasterController's Add Sim function, then I reset Olive Kipling. It showed as "nobody home" when my Sim rang the bell, but after she buzzed them at the callbox, I am able to ring the doorbell and talk to Olive if she's at home. Thank you so very much PuzzlezAddict!!!"

@kwohld You're welcome. I hadn't realized you'd wanted to assign a specific sim to an NPC door. (Some players want to add a second playable household to an apt building), so I wasn't sure what you were asking). Anyway, glad you got it sorted out. :)