Hi! With "añadir" you can say: "I'm going to add an apple to my cart" (voy a añadir una manzana a mi cesta de compra). "Agregar" you will use in the context of a facebook request, for example: "I'm going to add my new friend on facebook" (voy a agregar a mi nuevo amigo en facebook). In a way "añadir" is adding quantity to something or to incorporate something into something else, whereas "agregar" is adding but also accepting something new is happening in the process. Hope that helps!