Is it permissible to pray behind a Sufi imam who circumambulates graves, or behind his Sufi students?

Circumambulation (Tawaf) is an act of worship legislated only for the Ancient House (the Kaaba), by the consensus of Muslims. Circumambulating anything other than it is impermissible. Whoever believes that this is a form of worship and an act of drawing closer [to God] should be informed that it is not part of the religion. If he insists, he should be executed for denying what is known by necessity to be part of the religion.

Whoever circumambulates a grave seeking to draw closer to Allah is an innovator (Mubtadi') and misguided, and must be informed that this is not part of the religion. As for whoever circumambulates it out of humility and worship for the occupant of the grave, he is a polytheist committing major shirk, which expels him from the fold of Islam.

An Imam who circumambulates a grave seeking to draw closer to Allah (an innovator) - prayer behind him is valid if refraining from praying behind him leads to abandoning Friday prayer and congregational prayers. However, an Imam who circumambulates a grave out of worship for its occupant (a polytheist) - prayer behind him is not valid under any circumstances.