In Congregational prayer (Salah) while the Imam is reciting Fatiha, the followers should recite it at the same time or after he finishes it and say Amen?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

Reciting Al-Faatihah (Quran chapter 1) is one of the pillars of the prayer and it must be recited in every Rak'ah. The Prophet said: “The prayer of a person is invalid if he does not recite Al-Faatihah.” [Al-Bukhari and Muslim]

Al-Faatihah must be recited [it is a Waajib] by the Imaam and the person who prays individually alike, as well as by the people led in prayer according to the preponderant opinion of the scholars because of the previous Prophetic narration.

Besides, the Prophet said: “Perhaps you are reciting the Quran behind your Imaam. They replied ‘yes’. He said: “Do not do so, except Al-Faatihah because whoever does not recite it, his prayer is void." [Abu Daawood]

The people led in prayer should recite it when the Imaam observes a short period of silence after uttering Aameen. This short pause is desirable so that the people led in prayer could recite it, and even if the Imaam does not observe a pause, they should recite it.

Abu Salamah ibn ‘Abdur-Rahmaan said: “The Imaam observes two pauses: first when he starts the prayer, and secondly, when he finishes Al-Faatihah, so you should exploit this opportunity for reciting it.”

Allaah Knows best.