Think of a circle drawn around everyone who uses one particular thing. The thing in the middle is a machine, a program, a service. The circle is the group. Once the circle exists you can do two useful things with it: the people inside can talk to each other, and anyone outside can deal with the whole circle at once instead of with each person.
可以想成：一樣東西擺在中間，然後在所有「用它的人」外面畫一個圈。中間是機器、軟體或某種服務，圈起來的就是 user group。圈一畫出來，就有兩件事變得方便：圈裡的人可以互相講話、交換心得；圈外的人（廠商、管理者）也可以一次對整個圈做事，不用一個一個處理。後面幾個意思，都可以看成這個圈被用在不同地方。
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