Yes, you can - subject to availability. Rates are based on the number of guests in your reserved stateroom. Changes in occupancy will result in a rate increase or decrease, depending on the type of change. No refunds will be issued for any decrease of rate change until the stateroom has been paid for in full.
While many staterooms can accommodate more than two people with a sofa bed or upper berths stored in the stateroom ceiling, etc., not all rooms can; and such accommodations are subject to availability at the time of reservation. Due to Coast Guard safety regulations, there cannot be more guests accommodated in a stateroom than the designated occupancy for that room. Also, please be advised that each guest must be registered and paid for in full in order to be allowed to board the ship.