If you clear the check box in the 'Bibliographic Contributor' field in the contributor's permission settings, the contributor will no longer be a 'Bibliographic Contributor'.
The contributors who are not listed as bibliographic contributors can still access the project; however, they will no longer appear in the project's main screen, components' management page or contributor's list in the citation.
You can use this function for those who are helping to manage the project, but do not have a copyright on the project.