As you expand your base, you'll get brand new inhabitants wishing to join your vault, and they'll literally line up at the front door and wait to be let in.
You don't have to let them in of course, but the more the merrier. You'll be able to immediately put them to work and start improving and expanding your base, so don't hesitate to let people in and start expanding things.
Room for more
You'll need more space to be able to let people in though, and as such getting new living quarters and larger spaces for producing supplies will become important quickly.
Naturally, you'll need to get to work on buildings rooms for facilities. You can link three rooms of the same type together, and you should plan how you building things around that fact.
You can upgrade rooms too, making them more efficient and allowing you to fit more people in. You will also be able to change room themes at a point.
Everyone gets a job
There's no good reason not to put people to work. Well, they'll need some time off every now and then, and maybe to get social with the opposite sex, but otherwise? Work.
A large population requires plenty of supplies, so make sure that everyone is working on creating essential supplies and is working as efficiently as possible.
Many characters will be particularly proficient in a certain type of job, so assign them in whichever room they work most efficiently.
Birds and the bees
Love makes babies. Also you need living quarters, and two people of the opposite sex that get along with one another.
With enough time together, they'll go into the back together, snuggle a bit, the lady will be pregnant. Oops!
It'll take a few hours for a baby to come out, but once they do, congrats! There's no child labour laws here! Yes!
Going into labour is good, because it creates more inhabitants. Labouring is good, because it provides resources for your comrades. So why not combine the two?
Yes, pregnant women should be put to work. Get them in the kitchens, the power plant, the med bay, all of it, and have them producing the future of your civilization.
One thing to note though, is that they won't help in case of an emergency. Hey, they have to look out for two. As such, they won't be any good in a fire, or if enemies invade, so make sure everyone else has a weapon and is around to solve issues.