Comrade Chen will address the meeting. 陈同志将在会上讲话。
He addressed his friend on this matter. 他对他的朋友讲了这件事。
He addressed the rally. 他在群众集会上讲了话。
如果address 的宾语不是人或人的集会,其意思就不是“讲话”,而是”写“或”提出“。例如:
I want to address a letter to my friend. 我想给我的朋友写一封信。
I addressed a question to him. 我向他提出一个问题。
Speak to 是“向……讲话”的意思。Speak at是“在……会上讲话”, speak about (on) 是“谈……问题”。例如:
I'll speak to him about the matter. 我要同他谈谈这件事。
He is going to speak at the meeting this afternoon. 他将在下午的会上讲话。
He is going to speak about the international situation. 他将谈谈国际形势。