For the dragoons joining...
Well, first, the dragon have to find a human worthy for him...
The dragon only one person in the world he is able to link with, the one he found worthy...
Then they both have to pass some tests setted by the elder dragoons..
When they pass the test, the dragon and his rider have to cut their flesh and let their blood falls in a cup of water, mixed with some magical thingy...
After, they use the blood mixed with blablabla as Ink to make a tattoo on the dragon and his rider. The rider and dragon can choose the color of it. This links the dragon and his rider by their mind, soul or whatever you call it. The tattoo is chosen by the dragon and the rider, and they're name are written in an ancient language on it with a black ink.