This is pretty common amongst friends. The person who calls "shotgun" has the privilege of riding in the front passenger seat (ie. the shotgun position). If a group of friends are walking towards the driver's car, the first one to call shotgun gets to ride in front. In your example, the person throws the keys to another person (="you're the driver") and calls "Shotgun!" (= "I'm in the front passenger seat."). If there were only two people, then this would be a little comical.

http://en.wikipedia.org/wiki/Riding_shotgun Have a look at this link which explains the practice of "riding shotgun". I am guessing that this is the context of "shotgun" here. However, a front-seat passenger usually "rides shotgun", not a driver, so I don't quite fully understand why the passenger was thrown the car keys in this situation.

Also, the position of sitting next to the driver, derives from the American West, when the horse-drawn stagecoach carrying passengers and a "money box" required a guard who carried a "shotgun" to protect from outlaws. The shotgun was also called a "coach gun" was usually the Double Barrel type. http://www.cimarron-firearms.com/1878-coach-gun-12-ga-20-in-1878cg.html At close range, it was difficult to miss what the guard shot at. The shotgun was lethal at close range. http://images.search.yahoo.com/images/view;_ylt=AwrB8ph1DlJU_kwAbF.JzbkF;_ylu=X3oDMTIzazRhZXFpBHNlYwNzcgRzbGsDaW1nBG9pZAM0NWU5NmEwODhiZDE4ZGM0M2Y0ZTI2NDdkZGQ0ZTNlYgRncG9zAzMyBGl0A2Jpbmc-?back=http%3A%2F%2Fimages.search.yahoo.com%2Fsearch%2Fimages%3F_adv_prop%3Dimage%26va%3DStagecoach%2B%252B%2Bguard%26fr%3Dmcafee%26tab%3Dorganic%26ri%3D32&w=338&h=450&imgurl=www.guns.com%2Fwp-content%2Fuploads%2F2012%2F12%2Fcoach-shotguns1.jpg&rurl=http%3A%2F%2Fwww.guns.com%2F2011%2F12%2F12%2Fshotguns-the-guns-that-really-won-the-west%2F&size=42.4KB&name=when+an+armed+%3Cb%3Eguard%3C%2Fb%3E+rode+beside+the+driver+on+stagecoaches&p=Stagecoach+%2B+guard&oid=45e96a088bd18dc43f4e2647ddd4e3eb&fr2=&fr=mcafee&tt=when+an+armed+%3Cb%3Eguard%3C%2Fb%3E+rode+beside+the+driver+on+stagecoaches&b=0&ni=21&no=32&ts=&tab=organic&sigr=12aj5fdi1&sigb=13gtnbp4s&sigi=11r93f31b&sigt=121b3t08v&sign=121b3t08v&.crumb=xWHOE54PqWg&fr=mcafee