Combat > Alliances

Alliances
When a single guild is not enough, it’s time to form alliances. An alliance is a group of guilds that can freely visit each other’s guild halls, can easily identify one another in town, and share a single alliance chat channel. Alliances take their name from the founding guild, and the leader of that guild is also the leader of the alliance. No more than ten guilds can be in an alliance. Alliances can gain faction points based on the actions of their members; they also represent an even larger group of familiar players with whom you can go adventuring. In Factions, alliances are aligned with either the Luxons or the Kurzicks.

Forming an Alliance
An alliance leader (or a guild leader who wants to become an alliance leader) can invite other guilds to join an alliance. The guild leader sending the invitation handles this through the in-game user interface; and the alliance leader (presumably with a little help from fellow guild members) pays 1,000 gold pieces to bring the invitees into the alliance. An outside guild cannot ask to join an alliance—invitations to join an alliance only come from within the alliance. Any guild leader can voluntarily remove his guild from an alliance at any time.
http://eu.guildwars.com/gameplay/article/alliances1/