Users allowed to take a quiz

If you select uses at quiz settings it should allowed only selected users to play the quiz.
Please create support ticket and send us admin login details in it. We'll check and get it right.

Please help, Im confused here.
When I go to the Quiz details and move to the Access tab
I see 2 settings "Acess level" and "select users"
When i select the access level as Registered and select some users under it the selection of the users doesnt seem to have any effect. All registered users are still able to take the quiz.

I want only the users I select to be able to take the quiz at the frontend.
What will be the best setting to achieve this?