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Group of guilds
A group of guilds is just that: several guilds that have grouped together for some common purpose. Usually formed to fill out 40-man raid groups, so PUGs of such a large size are avoided. The politics of a group of guilds can be complicated and require a united leadership to be effective.
Not a commonly used term but one that encompasses such terms as "guild alliances", "guild groups", or "multi-guild groups".