Is it more correct for a worshiper, upon joining a prayer with an Imam and one other congregant, to ask the congregant to step back before joining the prayer, or to join the prayer and then alert the congregant to step back?

If a worshiper finds an Imam and another worshiper (Ma'mum), he should make the opening takbir behind the Imam, then pull the Ma'mum towards him. This is the correct approach, based on the Hadith of Jabir in Sahih Muslim, where the Prophet, peace and blessings be upon him, turned Jabir and then Jabbar to stand them behind him. As for those who believe in making the opening takbir to the left of the Imam first, or asking the Imam to step forward, they have deviated from the correct approach. This is because the Prophet, peace and blessings be upon him, did not approve of someone making the opening takbir beside him; instead, he pushed him backward. The Companions understood from his action what was correct, as narrated about Ibn Umar with Umar ibn Al-Khattab. The disagreement here is concerning what is more complete and preferable, but the prayer is valid in all cases.