Although it may not be required, as there are situations where passengers in a given stateroom are not traveling together, it certainly is expected and typical, especially for those on the same booking by stateroom. And if someone is arriving at a later time, the agent would typically ask those checking in about the others to confirm the original booking.
Groups perhaps not so much. But the check in for each stateroom is not considered "closed" until all parties actually boarding have checked in and notice has been provided for those not sailing.