Create a guest user by typing his email address in to the sharing dialog. The guest will receive an email invite with a link to create an account. He has only access to files which are shared with him.
Furthermore, the administrator has to whitelist the applications that guests can use. By default settings, avatar, files_external, files_trashbin, files_versions, files_sharing, files_texteditor, activity, firstrunwizard, gallery, notifications, password_policy and oauth2 are allowed.