The friend reward system is automatic - you get a reward when a friend of yours plays a game after you have played. When you are eligible to receive a reward you will see a sign above your friend's picture. You can claim it by tapping it.
The best way to ensure both you and your friends receive rewards is to be active in the game.
There is a limit to the number of rewards you can receive each day. You can get 2 rewards per day from a single friend and 8 rewards per day in total.
