As the characters stories begin to intertwine within the game, they'll actually encounter each other in the world. When this happens, they'll join up with the character you've chosen to play as and fight alongside you. They'll be controlled by the AI, so your input to them will be limited. You'll be able to give them basic cues, but that's about it. Incidentally, if you choose to play through the game a second time, you'll be able to control the way they develop allocating points into their skill trees. For what it's worth, the enemy AI seems like it'll be pretty robust, which can only mean good things for the battleworthiness of your allies.