Send users a welcome message (IM) from the slackbot when they join your Slack team. Uses https://github.com/smallwins/slack so look at that first. Load this somewhere in a running Node JS app.

slack.js

var slack = require('slack');
var bot = slack.rtm.client();
var token = "your_slack_token";

bot.team_join(function(obj) {
  console.log("Team join triggered");
  slack.chat.postMessage(
      {
          token: token,
          channel: obj.user.id,
          text: "Hello new user and welcome to the team!"
      },
      function(err, data) {
          // Handle errors here
      })
  ;
});

bot.listen({token: token});

Which dependency do I use in package.json? thanks

Check out https://janitr.net

thanks @kbariotis