Under the Manage tab, hover over Account Admin then select Manage Users from the pop-up menu.
(Note: this is only accessible to users with an External Admin account type).
From this control panel you can add and edit users, modify their access level (External Admin, External Regular, External Read-Only). You can also click on their corresponding row in the Shared Campaigns column and choose which campaigns they have access to, as well as which daily HTML reports they receive.
Watch this video for instructions: