This will be the start of a new organization: Defenders of the Hidden World. Our job will be to protect the Annex and protect dragons everywhere. Once the students of the School of Dragons learn enough about dragons to be able to defend them, they'll join us.[src]
After the Dragon Riders defeated Grimmel the Grisly at New Berk, Astrid decided to form new organization called Defenders of the Hidden World. Its purpose is to protect the Hidden World Annex that has opened on School of Dragons from those who may want to hunt dragons inside it. Astrid states that every trainer who is able to fight for dragons can join the organization.
It is highly probable that the original Dragon Riders are part of this organization.