Account Users belong to a reseller account. They can be given access to any website with the Account, and you have control over what permissions they have. An Accounts User can have access to as many or few sites as you wish. In addition, they can have View Only access on one site, and Update Theme access on another, as you would for a Site User. 

Click on Add User within Account Management to add a new account user to your site. 

In this case the user can only work on existing sites, as they are not able to create or destroy sites, or manage the account billing.