The Grand Companies of Eorzea are a military alliance of nations united in common cause to avert imminent catastrophe from engulfing the realm. There exist three Grand Companies: the Maelstrom, which sails out of Limsa Lominsa; the Order of the Twin Adder, which makes its nest in the Twelveswood; and the Immortal Flames, which burns in the heart of Ul'dah. Adventurers may freely choose the organization to which they would swear allegiance, regardless of nation of origin.

Company Name	City-state	Headquarters
The Maelstrom	Limsa Lominsa	Maelstrom Command
The Order of the Twin Adder	Gridania	Adders' Nest
The Immortal Flames	Ul'dah	Hall of Flames

Enlistment Process

To enlist with a Grand Company, first make your way to its headquarters, which can be accessed from the host city-state's market wards entrance. Once inside, speak with the recruitment officer and you will be asked to undertake a mission,* the completion of which will earn you induction into the organization as a recruit.
* Available only to adventurers rank 22 or above.

As recruits are not yet bound to an organization, adventurers are encouraged to use this opportunity to discover firsthand with which company's ethos they most closely identify.

Missions and Rewards

As a recruit, you will be asked to undertake a selection of missions, the completion of which will earn you rich rewards. The nature of the reward will vary based on mission difficulty and your contribution to the organization. Prove your worth by unfailingly completing the tasks assigned, and you will climb the ranks and gain access to all the benefits associated with higher standing.

Missions for recruits will be available in patch 1.18, with those for higher ranks scheduled to be added in 1.19 and beyond.

Grand Company Seals

Completing a mission will earn you an amount in Grand Company Seals commensurate with the difficulty of your assigned task. Each organization issues its own unique type of seal (see below), which serves as a currency of sorts and can be used to redeem myriad useful items.

The Maelstrom: Storm Seals
The Order of the Twin Adder: Serpent Seals
The Immortal Flames: Flame Seals

* Grand Company Seals are honored only by the organization of issuance.

Adventurers may check the amount of seals in their possession by selecting Attributes in the main menu, and then clicking the Grand Companies tab.

Source: The Lodestone