Q: Should one immediately say Amin after the Imam finishes or wait and finish reciting the fatiha after the Imam and then say Amin?
A: Saying ‘Amin’ after al-Fatihah is recommended. It is pronounced with an elongated ‘a’ sound and if the ‘mim’ is doubled (Am-min) the prayer is invalidated.
After concluding al-Fatihah there should be a brief pause before saying ‘Amin’ to distinguish it from the Quran.
It is preferred that the Imam and the follower(s) say it aloud together in an audible prayer as it is for one praying alone. If the Imam forgets or says it quietly, the followers should say it aloud.
Source: Sharh al-Muntaha