Since everyone requires their own separate account, you will need to sign up for the trip individually. There will be a space on the application to indicate your roommate request, and you can also inform your Trip Consultant so they can make note on your account (see "Flights" section for more info).
If you're traveling with a group of 3 or more, awesome! Appoint one person to fill out the application first. They can then go into their online account, hit the "Earn Travel Credit" tab, and give everyone else a special URL that, when used, will link up your accounts and give you all a discount as your trip approaches.