as @bartv already mentioned there is no possibility of access control within the community-edition, however accessing your local instance via the web / from outside can be achieved.
As for credentials: passwords or OAuth informations are not directly visible within the web-interface but they may be exploited using an n8n-API-key and the export-functionality.
on the other hand: I usually set-up a workflow for my clients using my local / web-based n8n-instance and credentials for a test-environment and then export the final workflow to my clients instance where they just need to assign the correct set of credentials.
There are multiple ways to have a working solution in your case.