How to use the threads.participants function in threads

  dim_person = messages.dimension(function(d) {return String(d["threads.participants"]);})
  dim_sender = messages.dimension(function(d) {return d.sender;})

dim_date_tt.filterRange([find_date_min, find_date_max]);
    dim_time_tt.filterRange([find_time_min, find_time_max]);
    var message_tooltip = messages.allFiltered()

    dim_date_tt.filter();
    dim_time_tt.filter();

    if (message_tooltip.length > 0 ){
      var full_date = String(message_tooltip[0].date);
      var full_time = String(message_tooltip[0].time);

      md_sender.select("p").remove()
      md_sender.append("p").attr("class", "md_text").append("text").attr("class", "md_text").text(message_tooltip[0].sender);

      md_conversation.select("p").remove()
      md_conversation.append("p").attr("class", "md_text").append("text").attr("class", "md_text").text(message_tooltip[0]["threads.participants"]);

      md_message.select("p").remove()
      md_message.append("p").attr("class", "md_text").append("text").attr("class", "md_text").text(message_tooltip[0].message);

      md_datetime.select("p").remove()
      md_datetime.append("p").attr("class", "md_text").append("text").attr("class", "md_text").text(full_date.substring(0,16) + full_time.substring(17,33))
    }

  })
}