I think it should not be optional to accept or not inmigrants, It would be better if they just came in, althou there should be an option to choose manually which citicen to accept, another to auto add them to your group and another to not accept any.
why is this important?
it will add difficulty, a growing population adds dificulty because you will be forced to provide food and shelter to as many as you can, otherwise they will start to riot and become agresive towards another in order to fulfill their needs.
in other games you see for instance groups of people comming to join other groups to have a better chance to survive than been on their own in the wilderness.
some inmigrants may be spies or members of enemy groups trying to steal from you.