@faizanzahid Those in the screenshot are the global privileges. You want to change the category privileges, selecting each category (see up to the right), at least the root ones. There is also a button to copy privileges to the children categories.
Those userrs are stored in groups with special names, here are the names assuming the category id is 1.
You can get the user ids from these groups by.
Groups.getMembers('cid:1:privileges:find', 0, -1, callback); This will give you all the user ids that have the find permission for category 1.
If you want to get their basic info like username picture slug so you can show user icons you can just use Groups.getMemberUsers('cid:1:privileges:find', 0, -1, calllback);