In order to add user licenses, click on the Team area in the left sidebar and afterward select the Members tab.
Note that your role has to be Owner/Manager/Billing to add a license.
- Click on the button "Add Licenses" to the right of the screen as shown below.
- Select the number of additional licenses you need and click "Add Licenses".
The price shown will be prorated for the number of days left in the current month. You can also select to purchase a yearly license for a discounted overall price.