In 3v3 or 5v5 mode, you can set markers on the mini-map to direct your teammates to attack or defend a certain location.
Below the mini-map in the upper right corner of the screen is a round icon. Tap this icon to toggle it between Sword and Shield--"Attack" and "Defend", respectively.
To place the marker, tap and hold on the Sword or Shield icon. A larger view of the mini-map will appear in the center of the screen. Now you can drag the orange indicator to the location on the map that you want to attack or defend.