Open the collection you want to give access to and use the submenu to go to Access. From here you'll see an overview of all the groups that have access already, you can revoke access from here as well as add new user groups to the collection.
You can also go via the user group to see what content each group has access to
Access can be prevented on each item, items that have been set to 'Prevent access' will show to users as 'Coming soon'.
You can also set quizzes to 'Disable add to study'. This will stop users being able to add the quiz to their Study but provided you haven't set it to 'Prevent access', they will still be able to take it as a standalone quiz.
Both of these settings can be reversed via the same drop down menu