atleast my groups always had a struggle on it, esp on heroic - so what we found out was: if a player looses his dragon, he can get a new one and continue fighting in one of the following ways:
A - Die, run back, mount up and fly up!
B - Jump down (AVOID PLATFORMS!!) and you'll find yourself just outside the instance portal, just enter and mount up again!
C - Put yourself out of combat in any other way! (Vanish, invis, FD(?))
Also, I found that I had problems outhealing the damage with only one green dragon, so what I do is I just stand on the ground and heal normally, while if someone dies they run back with a fresh new dragon - and we claim our profit!