On boss levels, invite friends by tapping the Invite Friends button. You’ll receive bonus Nibblers to start the level with when you invite friends to play.
You can also invite friends from the map screen by tapping the invite button below the mailbox. There’s an added benefit to inviting and playing with friends, as you’ll receive rewards in the form of coins and permanent extra lives when your friends start playing. See how many friends already play and which amount of friends will give you the next reward by tapping the invite button.