It’s not obligatory to say Ameen when someone is supplicating around believers unless they ask for it.

وَ سَأَلْتُهُ عَنِ الرَّجُلِ يَدْعُو وَ حَوْلَهُ إِخْوَانُهُ يَجِبُ عَلَيْهِمْ أَنْ يُؤَمِّنُوا قَالَ إِنْ شَاءُوا فَعَلُوا وَ إِنْ شَاءُوا سَكَتُوا فَإِنْ‏ دَعَا بِحَقٍ‏ وَ قَالَ‏ لَهُمْ‏ أَمِّنُوا وَجَبَ عَلَيْهِمْ أَنْ يَفْعَلُوا

(In a lengthy Hadith) And I asked him^asws (6^th Imam^-asws) about the man who supplicates and his brethren are around him, is it Obligatory upon them to say ‘Ameen’? He^asws said: ‘If they so desire to, they can do so, and they like to they remain silent. If his supplication is rightful but if he says to them, ‘Say Ameen’, it Obligates upon them that they do so’.[1] (an extract)

[1] بحار الأنوار (ط – بيروت)، ج‏10، ص: 271