In the gamemember

Alliance

Links your guild to allied guilds, with chat, bulletin and crafters in common.

In your game: Alliance

What it does: An alliance joins several guilds: an alliance chat channel, a bulletin where each guild posts notices, the allied guilds' crafters in the search for who makes an item, and the events each guild shares. It has four sub-tabs: Bulletin, Chat, Overview and Management.
Who sees it: Officers always see the tab; members, only when their game is already in an alliance. Bulletin and Chat only appear with the guild in an alliance. Management is officer-only, and posting on the bulletin or inviting guilds belongs to the ambassadors.
Where to find it: In the window (J or /gos), Alliance tab. Founding, joining and inviting are in Management. Switch it on and off in Settings › General › Modules.

An alliance has up to 16 guilds, and each guild up to 8 ambassadors.
The tag has up to 12 letters and numbers, no accents or spaces, and becomes uppercase.
The alliance chat is a game channel named GOS plus the tag.
The join code has 8 characters, without O, 0, I and 1 so they are not confused. A guild that has the code joins without waiting for anyone to approve.
An invite reaches the officer as an Accept or Decline popup, which disappears after 60 seconds.
On the bulletin, each notice has up to 200 characters, each guild keeps its 20 newest and only deletes its own.
A calendar event only goes to the alliance if Share with the alliance is ticked on it. It comes unticked.

The alliance channel is public: anyone who guesses the name can join. Do not put anything sensitive there.
Blocking a guild applies only to your game, and nobody is notified.
In Overview, the seen count only counts whoever has the addon and is visible: it is a minimum, not the total online.
A member who receives an alliance invite only sees a notice in chat: accepting is up to an officer.