You can easily add a user by going to your [My Agency] tab, and clicking on the [Add Agent] button. Simply enter the appropriate information and hit the [Add Agent] button.  As you do this, you will be reminded of the additional cost incurred (if this applies to your account).

If you need to adjust the "Role", click the "Edit" link beside their name, select the appropriate "Role" and save the changes.
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Also, if there is a user you need to "disable", you can click on the "Edit" link beside that login name, check the "Disable login for this agent" box, and save changes. When you do this, the cost for this login is deducted from your monthly payment.