The Sworn Council of Yliakum is such an organization.
Many thanks that you remember us ... but without Ogu, and a few lazy members, we couldn't stay active until today.
I think the Community of Vaalnor, the Organisation, or the Dark Empire are better examples of multi-guild organisations.
The Community of Vaalnor was only a single guild for a really long time. It split up once it basically died.
The Dark Empire is still only a single guild with four different departments.
I think a better example is DET. Many members in many different guilds, but the two associations are kept seperate from one another.