Is it obligatory for the congregant to follow the imam in the posterior prostration of forgetfulness (sujood as-sahw al-ba'di), even if the addition was made by the imam? And what is the ruling on one who did not follow the imam in this situation out of ignorance?

The latecomer follower (who joined the prayer from the beginning) must follow the Imam in the post-salaam prostration of forgetfulness (sujood al-ba'di) because the Imam is more knowledgeable about what necessitates the prostration. The follower's prayer is not invalidated by omitting it, whether intentionally, out of ignorance, or forgetfulness, but they would have abandoned a Sunnah (definition).

The post-salaam prostration of forgetfulness is not missed by a lengthy delay or by leaving the mosque. It can be performed whenever the follower remembers it, even after a long time. As for the earlycomer follower (who joined the prayer late), they should not perform the post-salaam prostration of forgetfulness until after completing their prayer and giving their salaam.