if the Ma'mum is one person, he should stand to the right of the Imam, as it was authentically reported in the Two Sahih (authentic) Books of Hadith (i.e. Al-Bukhari and Muslim) that
Ibn `Abbas (may Allah be pleased with him) said,
I once spent the night at my aunt Maymunah's house. The Prophet (peace be upon him) woke up to offer optional night Salah, so I woke up to offer Salah with him. I stood to his left, so he took me by the forelock and placed me to his right.
(Part No. 8; Page No. 20)
Agreed upon by Al-Bukhari and Muslim.
This is in case there is no one standing to the right of the Imam, as mentioned in the Hadith narrated by
Ibn `Abbas. However, if there is a person to the right of the Imam, there is no harm if the new comer stands to the left of the Imam. Everyone's Salah will be valid. However, it is a Sunnah to stand behind the Imam if possible, as the Prophet (peace be upon him) asked
Jabbar to stand behind him when they stood to his left and his right.
Related by Muslim in his Sahih (authentic) Book of Hadith.
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.