Whom did the White Council serve?

[Formendacil]

I think it's important to note that the White Council was not exactly a legislative body so much as a group of like-minded people who got together to plan strategies against Sauron. In line with what Inziladun is saying, I'd say that this like-minded group really only consisted of the Istari and the Eldar.

In the time of the Last Alliance, the Dúnedain would probably have belonged as well--but by the time the Istari showed up and the White Council formed, Arnor had declined into petty infighting and Gondor was myopically swallowing up the southern coasts--neither would have seen the big picture at all. Perhaps later, after the fall of Arthedain, the Dúnedain of the North would have been involved in the actions of the White Council, as allies and collaborators of Rivendell and Gandalf (ie. guarding the Shire), but I can't really see any of the rugged Lords of the Rangers sitting in council with them....