If you currently do not have the maximum amount of adult hominids in your clan, you may come across outsiders in a certain radius around your settlement. They will spawn whenever you create a settlement, change generation or evolve through the lineage.
In order for an outsider to become part of your clan, you must first befriend him by accomplishing a certain task. To find out what task you must accomplish, take a good look at the way the outsider behaves. This should give you a hint at what you must do to help them.