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You can allow specific users/roles/groups only to view and reply on your topic by going to the Permissions tab. By default, all users can view and reply on a topic.
To set the permission to certain users, you can enter their usernames manually;
to select users.
to select users defined by specific roles. When ticking Restrict who can view in this topic to or Restrict who can post in this topic to, if you set the * membership for the selected group, all users of that group will be restricted to only viewing or posting in this topic.
to select groups.
Then, click Add to perform adding the selected ones.
See here for more details.