Currently, Halo 3 does not have clan support. Unlike Halo 2, Halo 3 did not ship with it. The developers felt that this feature was not being used by enough people, so they didn't want to waste time including it in Halo 3.

Instead, they felt it would be more important to include Forge...the map editor. All we can hope for is that it will come with a game update. Till then, just party up.