The dragon riders would be children
Like other answerers have pointed out, a grown human is too heavy to be carried any significant distance, but a small enough child should be light enough.
Children have been used for all sorts of things that adults are too large to do. Probably the most applicable example is camel jockeys. According to the Wikipedia article, children as young as two years old have been trained as jockeys, all in the pursuit of the lowest possible weight. Camels run faster when unencumbered, and children can be bought relatively cheaply to replace those who grow up or fall off and break. Western examples of children in dangerous jobs include those crawling in narrow mines or under running machines in early textile factories. A somewhat nicer jobs is that of child singers.
If you include tasks that doesn't require small size or weight, the list of things children are used for gets much longer. Child soldiers are another horrible example.
You can teach very complex tasks to young children. Though not as good as teenagers or adults, the level of many child musicians, chess players, go players and circus artists is simply astounding.
As for the case in question: I see pterosaur-born, highly trained children being used to relay messages faster than any other messenger on Earth. I see them being used as unmatched watchers who keep track of vast areas, keeping the local authorities informed.
Edit for clarity: I think pterosaur domestication to a large extent would be an information technology. They could also be used for sport, like is done with the camels mentioned above.
In times of conflict, I see them as dangerous archers or sling operators as well as dropping darts and crude fire bombs to set buildings on fire. This will not defeat an army on its own, just complicate tactics for your opponent. They would stay high enough not to be threatened by archers on the ground, and would only have to watch out for other dragon riders (as I'd like to think early western observers would call them!). They could also land and have their pterosaur fight if the victim is alone and unarmed, or if there are many riders attacking together.
Early western explorers might not have been carried off by flying beasts, but they would likely have seen their ships set on fire. (This would also fit well with the idea of dragons breathing fire: You see something large fly high above, and suddenly the sail is ablaze!)
The dragon riders would also give the Maori unprecedented knowledge. They would not in themselves allow invasions of say Australian nations, but they would fly high above the world, gather news and make the best maps in history.
The careers of these children would be very short, and though they might be praised celebrities for a short time they'd have to find something else to do as they grow up. With careers that short I don't see this being a hereditary profession kept within certain families. The real significant profession might be that of those who train the riders, go around in search of new recruits and maintain a group of them.
