Step 1: Click on your initials located on the left corner of the screen, select " Settings and Billing":
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Step 2: On the Billing page under "Current Plan", adjust the number of active seats:

Step 3: Under "Settings and Billing" click on "Company Settings" and "User Management" here is where you can activate/deactivate seats:

Once all of these steps are followed, your new user can begin to send out active campaigns!