I made an apartment that is just a regular house with one room marked as an NPC room. That way, the NPC is basically a roommate. The rest of the house (except my sims' bedroom) is marked Public. I use the RBBB cheat so that my sim can upgrade the house as if he owns the whole thing and not just his room.

But the NPC just stays in their "room" all day, since NPCs hidden behind NPC doors get all their mood needs filled just by being in there. Is there any way to force them out regularly? I miss the Roommates from Sims 2.

Well, you could knock on his door. I'm guessing that was not the answer you were looking for

They are actually sent off world, so are not in the room at all. They might even materialise at other points around the world (for example, at the till they are assigned to) without you seeing them exit their room. So the only way to get them to come out would be to invite them out by knocking on the door, or using the call buzzer. (When you use the call buzzer, you'll see that the game has crammed about 10 NPC sims into that small room! )

I noticed that once my sim meets the NPCs, you tend to see them on the lot more often.

That sucks. I wonder if it's possible to mod the door to be programmable. Only allow X number of NPCs, don't alolow NPCs to teleport elsewhere, force the door to only provide the Energy motive...