It is fairly easy to overlook adding new contributors to the CONTRIBUTORS file that should exist in each repo.
We should augment the CI and/or our pre-commit checks to avoid that, here is a simple oneliner that would do the job (assuming a list of known SWH staffers is present in a given file, ~/swh-team.txt in the example):

git log --pretty=%an | sort -u | while read author ; do if ! grep -q "$author" ~/swh-team.txt && ! grep -q "$author" CONTRIBUTORS ; then echo "unknown author: $author" ; exit 2 ; fi ; done