To add a user you will need to first buy a seat for the user. Go to My Account > Account and Billing Information > Manage Account > click on your plan and then click "Edit Subscription". Choose the quantity you want to add. For example if you have 3 seats and need to add one more, make the quantity = 4. Then click "update subscription". Refresh the "Account and Billing Information" page. Look for "Seats (Users Allocated) and the "add" link next to it. Click "add" and fill out the form to add your new user. Once added, they will be sent a time sensitive email to complete their account setup so be sure they are ready and aware that this email is being sent to them and to act on it immediately.