An imam stood to offer a fifth rak`a in a four-rak`a prayer. He was reminded to sit by those behind him, but he would not listen. What is the ruling on those who followed his lead knowingly and intentionally?

If the imam stood to offer a fifth rak`a [cycle of prayer], those praying behind him should not have acknowledged it and the prayer of those who did so is considered null and void.
And God the Almighty knows best.