Only when you are assigned as an administrator, you can do this function. Normally, one category is created as public and it allows everyone to access without any restrictions. Meanwhile, one restricted category only allows specific users/groups/memberships to access. Also, you can assign moderators to your category and specify a set of permissions for members in the category.
1. Click
on the Forum Administration bar. The Category form will be displayed.

2. Enter the category title into the Title field which is required. Its length must be between 1 and 100 characters (Special characters and spaces are accepted).
In the Category tab, optionally you can also:
The first way
Enter names of users/groups/memberships that can access this category into the Restricted Audience textbox. The different values are separated by commas.
Use selectors next to the Restricted audience textbox.
i. Click
to select a specific user from the groups list.
ii. Click
to select all users with the specific membership in a group.
iii. Click
to select a group of users.
Optionally, select the Permissions tab to specify the category permissions.

Do the same ways as setting the "Restricted audiences" of a category.
3. Click Save to finish.