Sheikh ibn Bāz raḥimahullāh said:

“If the person being led knows that the imām has added an extra rak’ah, he does not stand with him, rather he remains seated and alerts him by saying: سبحان الله! سبحان الله! And the imām must return if two or more people notify him. He must sit down, recite the final tashahhud, prostrate two sujoods for forgetfulness, then give Salām. If he refuses and does not return, they must not follow him. They must remain seated and wait for him. Even the one who joined in must not follow him, rather he must remain seated like everyone else; because this rak’ah is a mistake, and they know that it is a mistake, so they do not follow him. If they follow him because they don't know the ruling, their prayer is valid. Even if they knew that it was an additional rak’ah, but they were ignorant of the ruling; maybe they thought that they have to follow the imām, then their prayer is valid. But if they know the ruling; that it is not permissible to follow the imām in this case and they still follow him, then their prayer is invalidated; because they deliberately added to the prayer... Likewise, in the case of ommission; if the imām remains seated in the third rak’ah of a four unit prayer and is alerted but he does not stand up, they must leave him and stand up and complete the remaining rak’ah for themselves. If the imām does not listen to them because he believes that he is in the right, his prayer is valid. And if he was unsure and did not stand up after being notified, his prayer is invalidated; because he is obligated to follow those who alerted him if there are two or more of them...So his prayer is invalidated in this case because he intentionally subtracted from the prayer - unless he believed that he was right in himself; if he believed that he was right and they were wrong, his prayer is valid. And if they believed that they were right and he was wrong, their prayer is valid. In such matters, they must all be careful so as not to fall into error. When one enters the prayer, he has to focus with his heart, pay attention and humble himself to avoid forgetting. But if one forgets, he follows the procedure set out in the Shari’ah. If it occurs from the imām, those behind alert him by saying: سبحان الله! سبحان الله! If the imām adds, they do not follow him, and if he decreases, they do not follow him - as long as they are sure that it is an addition or an ommission. So if they are sure of the addition, they sit and wait for him, and if they are sure of the ommission, they stand up and complete for themselves. And the imām must offer the prostration of forgetfulness if he increases or decreases, and they follow him in this prostration; if he he increases something and then returns back after being alerted, he completes the prayer and prostrates for forgetfulness, and if it was due to a decrease, he stands and competes the prayer for himself and prostrates for forgetfulness, and they prostrate with him.” Source: https://binbaz.org.sa/fatwas/13673/%D8%AD%D9%83%D9%85-%D9%85%D8%AA%D8%A7%D8%A8%D8%B9%D8%A9-%D8%A7%D9%84%D8%A7%D9%85%D8%A7%D9%85-%D8%A7%D8%B0%D8%A7-%D8%B2%D8%A7%D8%AF-%D8%B1%D9%83%D8%B9%D8%A9#:~:text=%D8%A5%D8%B0%D8%A7%20%D8%B9%D9%84%D9%85%20%D8%A7%D9%84%D9%85%D8%A3%D9%85%D9%88%D9%85%20%D8%A3%D9%86%20%D8%A7%D9%84%D8%A5%D9%85%D8%A7%D9%85,%D9%88%D9%87%D9%85%20%D9%8A%D8%B9%D9%84%D9%85%D9%88%D9%86%20%D8%A3%D9%86%D9%87%20%D8%AE%D8%B7%D8%A3%D8%8C%20%D9%81%D9%84%D8%A7