Does anyone know of a mod or other way to keep a door locked to everyone but maybe one sim, EXCEPT for when that specific Sim invites them in?

I recently downloaded some would-be apartments and it occurred to me that I may have to lock the front doors to each "apartment" to keep the neighbors (aka other household members) out... but what if the neighbors want to invite each other over? They wouldn't be able to get in! And I can't monitor everyone at once, when I'm watching over 4 or 5 different households.

(If it matters, my only EP is World Adventures.)

There are lots of door lock mods out there, but none that that I know of that let you then invite someone in past the door while it's locked. Why don't you just have a BBQ for the neighbors?
You could also use different doors for different areas of the house. For instance, I lock each bedroom and bathroom in a house for the sims that sleep in or use that room only. Then if I have a guest they can come into the main areas but can't get into the individual bedrooms. That way I can have the guest into the living room, kitchen, parlor, etc, as I please by just locking the appropriate doors before the party or invite. It's actually kind of like real life where they stay out of your bedroom out of courtesy and if you have a bunch of strangers coming to your party, you lock certain doors to keep them out. I often use the "Barrier to Entry" door from the Ts3 store for front doors. Not only can I screen entrants by multiple occult states ages etc and then have exceptions, but this door's locks work on any type of lot. Maybe something like that would work for you?
I only wish someone would mod that awesome "Barrier to Entry" door to have a better overall look and more looks to choose from or else add the features to all the in game doors.

Nraas Go Here mod has been updated to include door locks...

So this mod is still under testing and I'm not sure if it requires Nraas Story Progression to work or not, but the testing version of GoHere is basically the barrier to entry experience applied to all in game doors but even more extensively. I recommend you check it out because its really the only thing I know of right now that could come closest to fulfilling what you've specified http://nraas.wikispaces.com/GoHere+Testing.