What is the ruling on a Muslim asking another Muslim to commit an act of disbelief (kufr) as a joke, or asking him, as a joke, "Should I commit an act of disbelief or not?" And do these statements expel their speaker from the religion?

The command to disbelieve is an act of disbelief, whether a person says it seriously or in jest, because joking about a word of disbelief is an act of disbelief by consensus. As for the question mentioned in the query, its utterer does not become a disbeliever by it, because it might be an interrogative of denial, by which he intends to negate disbelief from himself.