After your repository is moved from https://github.com/Deltares to the new organization https://github.com/Deltares-research, the list of collaborators has been cleared. Each repository is assigned one administrator who is tasked with inviting new collaborators.
Inviting new collaborator can be done by the Administrator using the 'Add people' button.
For bigger repositories it is advised to user teams instead of adding individual users. At present it is not possible for administrators to create a Team. In such cases the administrator should send a request to the Github support group: github-support@deltares.nl