How to add Instagram Icon to the camera roll sharing on iPhone 5?

When I go to camera> then I select a photo > then I hit the share icon with the arrow> it pulls up mail, message, twitter, facebook, assign to contact, print, copy, and use as wallpaper. How do I add Instagram to that list (if possible)?

Added (1). FYI I already have the app, obviously… So don't tell me to download the app. The question is how to make the icon show up in camera sharing.

Download it from App Store on Apple in iPhone 5 or all types.