Which hook to allow specific users to add/edit/view comments?
There is no hook to do this. One possibility that doesn't require editing of the comment module is to change the comment status of the node depending on the current user using hook_node_load:

function MyModule_node_load($nodes, $types) {
    global $user;
    foreach($nodes as $node) {
        if(!user_has_access_to_node($user, $node)) {
            $node-&gt;comment = COMMENT_NODE_HIDDEN;
        }
    }
}


The hook will be called after the one in comment module, and will prevent users from posting comments unless user_has_access_to_node returns true.

The implementation of user_has_access_to_node would simply check if the current user is in the list of users allowed to edit the viewed node.