Yes, you can.
In order to book a whole cabin for you and your travel companions, you will have to select the same accommodation type for all passengers. If the 'whole cabin' option is available, please select 'whole cabin' from the drop-down list. In case the 'whole cabin' option is not available, you can select the 'single bed' option and if all passengers fit in one cabin, they will be placed in the same cabin automatically.
If you select the same accommodation type for all passengers, you should see the phrase 'whole cabin' in the trip summary (on the right hand side, just below 'Seat selections & prices'). This verifies that you are booking a whole cabin.