If all five of you take Smite that's 2,450 (5x490) true damage right off the bat. I believe Dragon begins with 3,500 health, so you can immediately drop him to 1,050 health. I don't think it will be very difficult to polish him off from there, no matter what your team composition is.
Mind you, I understand that your goal is simply to kill the dragon right when it spawns, and nothing else about your match is really important... right? So this, to me, is the path of least resistance to accomplishing the stated goal. Once you do it, you can probably work to improve efficiency by, say, everyone taking heal or developing your team composition.