In the administration, click on "Users" and look for the person to add as a submitter. It is possible that this person already created an account on the public interface of SONAR. In this case, she/he has no permissions to submit documents and are not affiliated to any institution.
These persons can be found with the facet "Missing institution"
If the user already exists with no permissions (role "user"):
The user is automatically affiliated to your institution.
If the user does not exist (after having checked, see section 1):