If you have already used up all the available seats that come with your subscription and need to buy additional ones for more devices, you can do so by following these steps. First, click on your display name > Account. In the Account section, click the Buy seats option and follow the instructions to purchase more seats.
If you are unable to locate the Buy Seats option, you can click on your display name > Contact Us. From here, use the Sales phone number to get further assistance with purchasing additional seats.