You'll get an email a day or two before the cruise telling you when the luggage has to be ready and when to check in for the bus. We took a
DCL transfer from POR and arrived at the port after boarding had begun, I think they were at our BG before we got through security. The next time we took a transfer from SSR and got to the port before it even opened....and waited forever to start boarding. I much preferred arriving after boarding had begun.