When two people want to pray in jama’ah and there is no third person available, the Imaam (leader) stands on the left and the muqtadi (follower) stands on the right, a few inches behind the Imaam and they start.
If a third person arrives after they begin their salaah, what should he do to join the congregation? Where should he place himself?
Nice Question.
If a third person comes and joins two who are praying, then the one who is praying behind the imaam should move back, because of the report(required content taken from large hadith) below,
Jaabir(R.A) said: “The Messenger of Allaah (peace and blessings of Allaah be upon him) stood up to pray, and I came and stood on his left. He took my hand and turned me around until I was standing on his right. Then Jabbaar ibn Sakhr came and stood on the left of the Messenger of Allaah (peace and blessings of Allaah be upon him). He took both our hands and pushed us back until we were standing behind him. -Sahih Muslim Book 042, Hadith Number 7149.
The scholars mentioned that if there is space in front of the imaam and no space behind the two people who are praying behind him, the imaam should move forward, but if there is space behind them and no space in front of the imaam, then they should move back. If there is space both in front of the imaam and behind the two people, the two people should move back and the imaam should not move forward, because he is to be followed and should not move, and because the imaam should pray with a barrier in front of him such as a wall or pillar, etc.
In congregation (Jama'at) of two men muqtafi should stand in line with the imaam, not in front of him or behind him, but perfectly in line with him, on his right.
When the third person arrives he taps on shoulder of muqtadi and stands behind imaam and muqtadi steps backward to align with third person and form a staff (row). This is usual practice, however, Imaam can also move forward and that is correct to. And allah knows the best.
