Showing comments only to same custom user role

Yes you can,

Frontend only

you would need to check each post comment author for it’s role. If the role of that comment author is not adverstisers then remove it from the array of comments attached to that post.

Then you would have to return only the comments if the current logged in user is also from the advertisers role.

using the comments_array filter we can go over each comments attached to a post. But this filter is applied in comments_template so it won’t affect the access comments in backend.

Here’s how you can do it

add_filter( 'comments_array' , 'wpse_filter_by_role_frontend' , 10, 2 );
function wpse_filter_by_role_frontend( $comments, $post_id ){

  $current_user = wp_get_current_user(); // retrieve the currently logged in user

  // go over each comments for the current post
  foreach( $comments as $key => $comment ){

    $comment_author = new WP_User( $comment->user_id ); // for each comment get the author user object

    // here we say unset the current comment if the role of the comment author is not the same as the role of the logged in user
    if( $comment_author->roles[0] != $current_user->roles[0] ){
        unset( $comments[$key] );
    }

  }

  // Return the filtered $comments array 
  return $comments;


}

Frontend and backend

add_action( 'pre_get_comments' , 'wpse_hide_for_backend' );
function wpse_hide_for_backend( $comments_query ){

 // Hide all for non logged in users 
 if( !is_user_logged_in() ){
    return $comments_query->query_vars['comment__in'] = array(0);
  }

  $current_user = wp_get_current_user();

  // if you don't want to apply restrictions to admins
  if( $current_user->roles[0] == 'administrator' ){
    return $comments_query;
  }

  $user_ids = get_users( array( 
    'role__in' => $current_user->roles, 
    'fields' => 'ID' 
  ) );

  $comments_query->query_vars['author__in'] = $user_ids;

}

EDIT

I modified the second function.

Seeing @birgire’s answer I realized there was an author_in query arg and using get_users with query arg role__in we can achieve the desired effect on both front and back end.

So thanks to @birgire for the inspiration 🙂

EDIT

To allow more than just the role of the current user in the fetching of comments (like adding comments made by administrators) simply append the array in role__in with the desired roles

so the function would become

add_action( 'pre_get_comments' , 'wpse_hide_for_backend' );
function wpse_hide_for_backend( $comments_query ){

 // Hide all for non logged in users 
 if( !is_user_logged_in() ){
    return $comments_query->query_vars['comment__in'] = array(0);
  }

  $current_user = wp_get_current_user();

  // if you don't want to apply restrictions to admins
  if( $current_user->roles[0] == 'administrator' ){
    return $comments_query;
  }

  $user_ids = get_users( array( 
    'role__in' => array(
      'administrator',
      $current_user->roles[0],
    ), 
    'fields' => 'ID' 
  ) );

  $comments_query->query_vars['author__in'] = $user_ids;

}

or (for readability on this forum)

$permitted_roles = array(
  'administrator',
  $current_user->roles[0],
);

$user_ids = get_users( array( 
    'role__in' => $permitted_roles, 
    'fields' => 'ID' 
  ) );