All account owners have the option to add additional sub-users under their account. The owner can pick and choose what website's to give these sub-users edit permissions to. They work great for delegating content editing to other people. Only website's using the premium plan are available to sub-users.
When a sub-user logs in, they will only see the premium website's the owner has allowed them to edit. They will also have limited permissions for those website's. They will not be able to purchase anything, add new websites or delete current ones. They should only be allowed to edit content.