while i’m perfectly happy with an approach of new citizens asking to join, based on the success of my town, i seem to recall something, somewhere about players being able to actively acquire units… but, the specifics escape me… 
this is the best quote i can find on the matter:
How will your settlement grow population wise?
“Your settlement will grow at the same scale as your food supply. We haven’t nailed down the final mechanic yet, but we want there to be a constant choice between burning resources to craft things (buildings, weapons, armor, etc.) and investing in the food supply (farming, trapping) to get more people.”