There is a difference between agent users and agent seats. You can add as many agent users as you like, but the number of agents that will be able to log in at the same time will be determined by how many agent seats you purchase. To upgrade your account to allow more agents to log in simultaneously, click on Upgrade in the top header and choose how many agent seats you want to add. Choose your billing preference and add to cart. Your seats will be added to your cart and you can proceed to checkout to make payment.