Fireteam

A Fireteam is a combat unit of Guardians formed to conduct infantry operations with tactical flexibility. Fireteams act similarly to clans in terms of game mechanics. Fireteams typically consist of up to three Guardians, though fireteams taking on a Raid may number up to six.

Gameplay
Fireteams are optional in story missions and on Patrol, but are required in Strikes and Raids. Players can form their own fireteams if they wish, but if they enter a strike alone, they'll be matched with players of similar rank playing on the same strike mission. Forming a fireteam is necessary if one wants to bring other players into a Darkness Zone. If all members of a fireteam are downed simultaneously, progress is reset.

Players can form fireteams in one of two ways: send an invitation, or public meetup. If a player has their Fireteam Privacy set to "Invitation Only", then anyone wishing to have that player on their fireteam must send them an invitation in-game via the player inspection menu. But if that player's privacy is set to "Public", any nearby player can automatically join. There are 2 chatlines that fireteam members can communicate through: Fireteam Chat, and Team Chat.

Trivia

 * David Dague replied to question on Bungie.net, The question being "what in-game devices will there be for clans?" Dague replied with "what are clans?". This joke goes further with the fireteam card file name being "ClansLol.jpg".
 * Jason Jones is Bungie's "fireteam leader."

List of appearances

 * Destiny