Go to the Manage Users tab within Account Management to manage account users for your site. 

Click on a user to view their details, including the account permissions they hold, and the sites they have access to. 

Click on a site to edit site permissions that user has for a given site. Make Owner, makes this person responsible for the site. Remove from Account takes this user off, and is no longer able to access any of the sites they previously had access to unless they are a Site User on those accounts. 

Edit User edits the account permissions for the Account User. 

In this case, the user is able to create new site, but cannot destroy accounts or manage account billing.