What is the ruling on pulling a worshipper who is leading someone in prayer to join the congregation, and does pulling him invalidate his prayer unless it is accompanied by an initial takbirat al-ihram?

The Sunnah for someone arriving while the Imam is praying and has a worshipper beside him is for the newcomer to stand to the Imam's left, then say the Takbir, then for the Imam to move forward or for them both to move back behind him. This is based on what Jabir narrated in Sahih Muslim. If a latecomer pulls a worshipper back and then says the Takbir, the prayer of the one who was pulled is valid. However, the one who pulled him has caused him to miss the virtue of being pulled from one row to another.