You can add an unlimited number of users to your account, providing you have purchased licenses for these users. Once you have purchased the licenses you can add your new users, or remove old users, from your Account Settings page, Choose Add/Edit Other Users, then click the + sign to add your new user and set the level of access they will have.
Purchasing Additional Licenses
On your Account Settings page, there is a link 'Add License'. By clicking this link you can purchase additional licenses for your account.