If you only make JanusGraph accessible to your users through JanusGraph Server instead of giving them direct access to your backends, then you can configure authentication via JanusGraph Server. This authentication however basically only restricts access in general to authenticated users. This is documented
in the JanusGraph docs where a special credentials graph is used that is stored in the storage backend and additionally
in the TinkerPop docs where the credentials are stored in a file that needs to be shipped with the server.
If you for example only want some users to have write access to your graph and only read access for others, then you can start different server instances where some only provide read access and have different users than the other servers that provide full access.