To add an additional plan for a new device or person, login to your account and click the "Account" tab.
Then scroll down to the section with the heading "Your Plans."
In the box that shows the plan you are currently on and the ones you're already paying for, you'll see a button which says "Add Plan."
Click that button and follow the steps to create a new account for a new plan! The new user will receive an SMS with their login instructions so they can login, activate their account, and verify their device(s) to be protected.
The plan cost will be bundled with your existing monthly/annual bill.