You can add seats to your Connect Fonts subscription from your Subscriptions page. Click on Manage next to your subscription information and select "Change quantity".
The Change Quantity page shows your current subscription amount. Under New Quantity, click the plus (+) button to add seats to your seat count: if you have one seat and want to add two seats, the new quantity should be 3. You can also enter the desired quantity directly.
You will see a prorated price for the seats, based on the time left on your current subscription. When you click Confirm you are charged for the additional seats and they are added to your subscription.