When installing Xcode from mac AppStore, where is the DMG?
in case you download Xcode from AppStore, Pablo is right.
However, in case you want the "good old" download of the .dmg file from Apple's website, you have to login at the Apple Developer Download website (previously called: ADC Program Assets). You will find various versions of Xcode under "Developer Tools".
It's usually under Applications folder. The program is called Install XCode. You can copy and use that in your 100+ workstations.
Here's a question on apple.stackexchange.com where you will find a similar problem/answer.
Once you are logged into apple developer portal you can navigate to following link to view all development resources available for downloads as .dmg file, it includes XCODE 5 too.