Instead of using "Amen" as Christians finish their prayer, is it also common in English-speaking churches (I'm from a Chinese-speaking church) to say "Amen" whenever believers feel strongly like to agree with others as they pray? I'm talking about this use as a response to other's prayer, and after a person's own spiritually encouraging sharing, as in:"We finally got a chance to talk about God there. Amen!" Thanks for the answer.
It is definitely not unheard that people say Amen to show agreement with someone else making a statement. For some, though, this might also carry the tone of "finally someone said the right thing". There might also be other ways that people will understand this.
Note in particular that your tone of voice and your expression when you say it can have an impact on how people perceive what you are communicating.
I guess that you can compare this to people saying "Amen brother". Saying this clearly shows that you agree, but I might not use this unless you are in a culture where saying such is acceptable.
As for writing Amen as a sign of agreement, I would not do it. People will not be able to hear your tone of voice when you write, and so some people might read religious undertones into what you are writing.
Note also that (according to Wikipedia) Amen means "so be it". This is, of course, the meaning that we use after, for example, a prayer. However, I don't know if saying "so be it" can be a sign of agreement.