Instructions
Right-click your character's unit frame at the top left of the screen. The unit frame displays an icon of your character and your character's health and mana bars.
Hover over "Loot." A window will appear to the right. Click "Group Loot."
Click "Need" or "Greed" when an uncommon or rarer item drops while in a group. "Need" indicates your character needs the item, while "greed" indicates your character could use the item, but doesn't need it for his primary build. If someone chooses "Need" and you select "Greed," your roll will not appear; as "Need" takes precedence.