Even though you are at one address, you and your roommate(s) each need your own separate SimpleBills accounts. When you create your SimpleBills account, you will have the option to send each of your roommates an invitation to create their account.
If they did not receive this invitation, they can go directly to simplebills.com and hit the Get Started button in the top right corner of the page. This will lead them through the sign up process. Once their application is processed, they will be added to your address and will be able to manage utilities with you!