Is it obligatory for one who is praying a supererogatory prayer after the Maghrib prayer to prostrate with the Imam and the congregants upon the recitation of an ayah of prostration, and then to complete their prayer, or is it not obligatory for them to do so?

A worshipper is not permitted to perform a prostration of recitation (Sajdat al-Tilawah) for a recitation by someone who is not their Imam. If they do so, their prayer becomes invalid. Therefore, the aforementioned worshippers are not allowed to prostrate for the recitation of the Imam and his congregation after the prayer. Also, the Imam should not recite the Qur'an aloud after the prayer, as it causes disturbance, in accordance with the Prophet's (peace be upon him) saying: "And let not some of you raise their voices over others with the Qur'an."