To enter user management menu go to My_site_name/wp-admin/users.php and you will see the following:
Press "Add New" button to add a new user. Fill up all required information (Username will be used for log in) and press "Add User"
To delete user push "delete" button near username and icon.
To edit user press "edit" button. Here you may change account details and password. don't forget to press "Update user" button.