If you have used all available licenses on your Enterprise plan and need to buy additional licenses, you will need to navigate to your Profile screen.
Select the "Subscription" tab on the left. And then "Update Number of Licenses" on the right.
When you open the "Update Number of Licenses" screen, you will be able to add the number of licenses you desire. Each license will cause an additional $14.99 to be added to your monthly billing.
Enter the number of licenses you wish to add, and then hit the "Update number of licenses" button. This will return you to the main subscription page and you will see the updated number of licenses available.