When in guild runs, this can be a great tool Battle Standard of Coordination
If everyone (or, say, at least two people) has one in your group you can put them down just before you kill a boss to give you an experience and/or reputation boost. Very useful when we need to cap the guild, too!
You can also use the lower versions of this one, Standard of Unity
(pictured), Banner of Cooperation
. They give you less, though.
Consult your Guild Herald
or Guild Page
or visit the nearest guild vendor!
Note: These items are bound to guild reputation - Friendly.