How to express among in chinese?
Well, sometimes you won't translate 'among', see your first sentence. Other times, 'among' will translate simply as 中, or a combination of 在 ... 中, like your second sentence. 在 ...  间 is also common, but there are other ways. Of course, exactly if and how 'among' is expressed will depend on the context.


He's very popular among the students.
他很受学生（的）欢迎。  
Quite a few students have passed the exam, and I am among them.
相当多的学生通过了考试，我也在其中。


Edit: Reading this morning, I came across this:

国家文物局明确指出，“西市” 是隋唐丝绸之路的起点，要纳入申请世界文化遗产的名录当中。

纳入 ...当中 or just plain 当中, in my opinion, is "among", but I wouldn't translate it that way.