If the imam or a person praying in congregation forgets to recite al-Faatihah, what should he do?

Firstly: we have already stated in the answer to question no

10995 that
reciting al-Faatihah is an essential part of the prayer without which the
prayer is not valid. It is essential to recite it, whether one is praying
alone, as an imam or behind an imam, and whether the prayer is one in which
Qur’aan is recited out loud or quietly. 

Secondly: If the imam forgets it in the first rak’ah, and he
does not remember until he stands up for the second rak’ah, then the second
rak’ah becomes the first rak’ah for him. Based on this, he has to offer
another rak’ah to make up for the first rak’ah in which he did not recite
al-Faatihah. With regard to the people praying behind him, they should not
follow him in this rak’ah, rather they should sit for the tashahhud, then
wait and say the salaam with the imam. 

With regard to a person praying behind an imam who forgot to
recite al-Faatihah, for those who say that the person who prays behind an
imam does not have to recite al-Faatihah the matter is straightforward: he
does not have to do anything. For those who say that it is an essential part
of the prayer for him, and he is like the imam, then if he did not recite
it, he has to offer another rak’ah after the imam says the salaam, unless he
joined the prayer when the imam was bowing, or when the imam was standing
but he bowed before he could finish reciting it. In that case he does not
have to recite it in the first rak’ah.

Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) ،
Liqaa’aat al-Baab il-Maftooh, 51.