What is the ruling on the Imam standing for a fifth rak'ah due to forgetting Al-Fatiha, and is it obligatory for the congregants to follow him, and is their prayer valid with the intentional addition of a fifth rak'ah?

If the Imam adds a fifth rak'ah, the congregants, according to the majority of scholars, should not follow him. Instead, they should either separate from him or wait for him to conclude the prayer with him, because the congregant's prayer is already complete. However, the Malikis and some scholars have permitted following if the congregant knows that the Imam is making up for a deficiency. These scholars may refer to what has been narrated from the Companions.

It is not permissible for the Imam to speak during the prayer to instruct the congregants to follow him, due to the generality of the hadiths that prohibit speaking during prayer, except for what has been narrated from some scholars regarding its permissibility for the benefit of the prayer, citing the Hadith of Dhu al-Yadayn, which is a subject of research and discussion.

The preponderant opinion is not to follow the Imam in the extra rak'ah, and that speaking during prayer is absolutely impermissible. However, one should not condemn those who follow the Maliki madhhab and some scholarly opinions regarding following the Imam and his speaking if it is to make up for a deficiency in the prayer, due to the conflicting evidences on this matter.