After you reach the end of Episode 1 you will be able to start swapping creatures in and out of your party by using the button next to your creatures.
Being able to swap different allies into battle definitely offers you a strategic advantage, but another benefit is that it will increase a swapped in creature's level to match the average level of its new party. Lastly, swapping a creature in does not cost you a battle turn like changing formation does, so have fun experimenting!