You can.
You simply add those that you want to have access to the "Members" group and give them Create_topic and Reply permission.
You also need to remove the View permission from the Anyone and Registered groups.
There's much more about the principles at:
http://greghelp.991552.n3.nabble.com/Administrator-s-Guide-f4012830.html
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.