We previously announced in the Cataclysm Beta forums that we'd be enforcing a 600 guild member cap with patch 4.0.1. We've reevaluated what we believe the realms are capable of supporting and instead will be enforcing a guild member hard cap of 1,000 members. This means that guilds with more than 1,000 members will no longer be able to invite new members until they drop below the cap. For example, if a guild has 1,500 members they'll be able to continue as-is, playing and earning guild achievements for as long as they choose, through Cataclysm and beyond, but they will not be able to add new members until they fall below the 1,000 member cap. If a guild has fewer than 1,000 members, they won't be able to add members above that number.
This new guild cap is being enforced for several reasons, but they all factor into a need to control guild sizes in light of the new guild systems, including guild leveling and achievements. Previously, adding players to a guild was as simple as adding people to a chat channel. While approximately 500 members were visible in the UI, there was no real need to limit guild size. That is no longer the case. Guild leveling in Cataclysm features unified progression powered by a series of complex systems that track the contributions of all guild members. The larger the guild, the bigger the impact on these systems. We found it necessary to determine a maximum guild size to ensure continued performance now and into the future.
We want to reassure the few guilds that this does impact that they have ample time to plan and rearrange their guilds as they see fit. While this hard cap will go into effect with 4.0.1, guilds should feel safe rearranging members as necessary up to the release of Cataclysm on December 7 when the guild leveling system goes into effect. There will be no impact on guild leveling progression before then. If a guild were to disband after Cataclysm, or a group of people were to leave and start a new guild, they would lose the time and experience they previously contributed and potentially extend the amount of time it would take to regain the new guild perks.
While some players have used options including mods and custom chat channels to support large player and guild alliances that number multiple thousands, groups of that size aren't ideally suited to our design philosophy. As always, we're continually looking into adding new features to help facilitate guild management, scheduling, and player communication.