I'm tired of being in organizations where even though people are speaking English, they do so with such a poor inflection that you can't understand them; or they speak as though they learned English from a chat room.
You'll probably find most of them on Chronos. That server used to be the NA-server.
Otherwise, I'm not entirely sure. Most clans I've stumbled upon are filled with people going "mader faker" and "hauahauahuha".
They might be awesome people once you get to know them, but for some reason I tend to avoid those people.
I hear Shinigami has a lot of americans. I'd check that out.
Also, Team Jesus has a lot of native English speakers.