What is the correct way to arrange the row if I enter the prayer with an Imam and one worshipper: Should I ask the worshipper to move back and then join the prayer, or should I join the prayer and then alert the worshipper to move back?

If a Muslim arrives and finds an Imam and one congregant standing, and wishes to join them in prayer, he should begin the prayer (perform takbirat al-ihram) to the left of the Imam. Then, either the Imam should move forward or the congregant should move back to be on the Imam's right, based on the hadith of Jabir (may Allah be pleased with him). However, if a second congregant arrives and the Imam and the first congregant are in tashahhud, no one should move forward or backward; instead, he should wait until they stand up.