Groups of guilds

From WoWWiki

Groups of guilds are 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 these groups 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".