What is the ruling on a Muslim joining a prayer congregation consisting of two men, and must the person being led in prayer (ma'mum) step back to pray beside the man who arrived late, or should he continue reciting next to the imam?

If a third person joins two people who are praying, they should move back from the Imam. If there is space in front of the Imam and space behind the two congregants, the congregants should move back, and the Imam does not move forward because he is the one being followed. If there is no space behind the congregants, the Imam should move forward. This should be done after the second congregant has entered into the state of ihram (begun the prayer).