Putting your registered members into member groups serves two different purposes: First, it enables the ability to restrict access to different categories on your forum based on a member's group assignment. Second, you can assign special "powers" to member groups such as the ability to lock topics or delete threads.
As the Administrator, you can create, edit, or delete existing member groups in your Administration Panel under "Member Groups." Creating or editing a group lets you specify the name of the group, the group's color, special powers, and much more.
To limit access to forum categories based on member group, first go to your Administration Area: Click on the "Modify Category" link then, on the next page, select the category you would like to change and click "Continue." Now you should see a section labeled "Limit Access"; this is where you can choose which groups can access this forum category. Select the boxes for the member groups you want to have access, leaving the rest unchecked. When you are done, click "Modify Category" at the bottom to finish.