All user configuration is done on the Manage Users section available in the Control Panel on the Dashboard or the Admin menu. And it's only available to account Admins.
On the Dashboard, under Control Panel > Click Manage Users
Or you can also click Admin > Manage Users
On the Manage Users page: Click either Invite or Create users
To invite with just an email address, or create the user entering all the fields required (Name, login, etc.)