Consumable: Fireteam Medallion
The strongest Guardians are those who stand together.
Obtained: You obtain one of these for completing the story for the first time but there is an alternate way to earn more Medallions. If you get lucky you will get a stack of Bright Dust from a Bright Engram. These are a currency used to buy items from Tess Everis. One item you can purchase from Tess is Fireteam Medallions for the price of 50 Bright Dust.
Usage: Increased XP gains and loot for you and members of your fireteam or match from strikes, Public Events, and the Crucible. Effect lasts for 4 hours. You can see the time remaining by opening the inventory, then to the right of your primary weapon it shows a green icon. Scrolling over the green icon shows the time remaining.
This is where you find Tess Everis in The Tower to purchase Fireteam Medallions: