当中 (dāngzhōng) means "among," or more literally "in the middle of." In English this is a bit like "of," as in, "of all of my friends, you are the most loyal." Also, 当中 can be used to express being "in the middle" of an activity or process that is fairly formal in nature, such as "in the middle of negotiations."
Structure: Group / Process + 当中
你是我所有朋友当中最聪明的一个。(nǐ shì wǒ suǒyǒu péngyou dāngzhōng zuì cōngmíng de yī gè) = Of all of my friends, you are the smartest one.
在这一百个选手当中我表现得最出色。(zài zhè yībǎi gè xuǎnshǒu dāngzhōng wǒ biǎoxiàn de zuì chūsè) = Among these one hundred contestants, my performance was the most outstanding.
他们当中很多都是上海人。(tāmen dāngzhōng hěn duō dōu shì Shànghǎi rén) = Among them, there are many Shanghainese.
五个人当中有一个人是小偷。(wǔ gè rén dāngzhōng yǒu yī gè rén shì xiǎotōu) = Of the five people, one of them is a thief.
在这么多家餐厅当中，就是他们家比较好吃。(zài zhème duō jiā cāntīng dāngzhōng, jiùshì tāmen jiā bǐjiào hǎochī) = Of the many restaurants, theirs is best.
这个工程还在计划当中。(zhè gè gōngchéng hái zài jìhuà dāngzhōng) = This engineering project is still in the planning stage.
这批产品还在设计当中，还没有完全确定。(zhè pī chǎnpǐn hái zài shèjì dāngzhōng, hái méiyǒu wánquán quèdìng) = This batch of products is still being designed and has not been finalized.
我们还在考虑当中，你不要着急。(wǒmen hái zài kǎolǜ dāngzhōng, nǐ bùyào zhāojí) = We are still in the process of considering it, please don't get impatient.
现在还在调查当中，还不知道结果。(xiànzài hái zài diàochá dāngzhōng, hái bù zhīdào jiéguǒ) = It still under investigation, so the result is not yet known.
我还在搬家当中，每天要处理很多事情。(wǒ hái zài bānjiā dāngzhōng, měitiān yào chǔlǐ hěn duō shìqíng) = I'm in the process of moving, and every day I have many things to deal with.
When used to express "among," 当中 is a little like 其中 (qízhōng). However, 其中 means something more like "among the aforementioned," where as 当中 simply means "among." Furthermore 其中 cannot be used in relation to processes.