Should one join the worshippers who have deviated from the Imam's position, or stand alone behind him?

The first and best is to stand behind the Imam, with him in the middle of the row. What the two individuals standing to the left of the Imam did is contrary to the Sunnah, and their prayer becomes invalid according to the Hanbalis if there is no one to the Imam's right. However, the majority of scholars hold their prayer to be valid. You are not obligated to stand with these two individuals; rather, you should stand behind the Imam, and your prayer will be valid, even though this posture is contrary to what is most preferable due to it breaking the row.