Integration bus is useful when it's to synchronize to (or from) a different system where it's important that failed tasks will be repeated and that everything is handled in the exact same correct order. For instance a custom connection to a CRM system.
If you want to handle the authentication (so are this credentials valid for logon) you usually let the external authentication system handle the data.
If you want to handle authorization (so is user X in role Y allowed to do/view Z) you can set up synchronism mechanism. Since external systems usually don't push this kind of data. You are probably better of building a custom scheduled task that periodically imports/updates user data. In this case the authentication (logon credentials) is handled by Kentico.
If you can specify what you want we might be able to help you more.