Hizb ut-Tahrir, a group so radical that the Muslim Brotherhood won't even touch it, lives and breathes in America. Part of the reason it's a fringe group in the U.S. is because it is mostly honest about its objectives. For example, one of its websites answers a question about the status of Christians in an Islamic State by frankly admitting that Sharia is imposed upon them and other non-Muslims.
"Everyone who carries the citizenship of the [Islamic] state enjoys the rights and is bound by Shari'ah obligations," the Hizb ut-Tahrir (HUT) website says.
The article later reiterates that the Islamic state "implements Shari'ah upon all of those who carry the Islamic citizenship whether they are Muslims or non-Muslims…"