Users must belong in User groups for which you can set permissions.
Joomla' default user groups (Users -> Groups). This is is a parent child relation. When you are setting permissions, please bare in mind that a child user group inherits parent user groups permissions.
By default there are 3 user groups that can access the administration backend. Super Users, Administrators, Managers.
You may change that if you wish except from the Super User account. You can create your own user groups according to your needs.
You can set permissions for each user group ( Users -> Manage -> Options -> Permissions).