Were the Sadducees a Jewish sect or a completely separate religious group? I read they only follow the Pentateuch and do not believe in resurrection? What became of the Sadducees?

Yes they were Jewish, and they only followed the Written Law and not the Oral Tradition. In Judaism, even if one is not religious or is mistaken religiously, he is still a member of the Nation of Israel, for we are not just a religion but also a nationality. After the destruction of the 2nd Temple, like all non-Orthodox sects, they eventually withered away.