Is it obligatory to join a congregational prayer of two individuals while they are prostrating, or should one wait until they stand or finish their prayer with the taslim? And if one joins, should one stand beside them or behind them?

It is required for a latecomer to join the Imam in whatever state he finds him, based on the Prophet's (PBUH) saying: "If one of you comes to the prayer and the Imam is in a certain state, then let him do as the Imam does." And based on his saying: "If you come to the prayer and we are prostrating, then prostrate, and do not count it as anything." This is what Al-Hafiz Ibn Hajar used as evidence, and Ibn Hazm agreed upon it.

If a latecomer enters and finds two men praying in congregation in the state of prostration, he should prostrate to the left of the Imam. Then, after the Imam stands, the two congregants should move back to form a row behind him.