Registrars are the organizations or individuals that are allowed access to the automated system of CoDNS in order to register domain names for their customers or themselves. Co.nl domain names can only be registered via Registrars that are accreditated with the operator. A full list of Registrars where .co.nl domains can be registered is viewable here.

If you are interested in becoming a Registrar you can review the full procedure, explained here.