How do I get an NPC to follow the player and then allow the player to swap between characters

I am making a game with two characters with differing abilities and want them to both follow the player's controls - like an NPC following the player. And then I want to allow the player to swap control between the two so that they can help each other.

I am using the third person controller from the sample assets for the main characer, whats the best way to get this done?

Cheers