To add new agent licenses,  navigate to Admin -> Account -> Account Details-> Plans & Billing -> Choose this plan where you could increase the number of agent seats on your account. 

Here, the number of agent seats would be displayed, which you could edit and click on "Proceed to Payment". After the payment is authorized, the agent seat(s) would be added to your account.

If your payment is offline, please write to, with a request to increase agent seats and the number of agents you are looking to add. One of the agents from the Billing Team would get in contact with you to process your request.