NodeJs : modules.export : Cannot find one of my own custom module

I am trying to include my own users.js module to my router file. It keeps throwing the error:

Cannot find module './router/users.js'

My directory structure is as follows:

nodejs (Main folder on my drive)

-- expressserver.js (My server file)

-- package.json

-- router (folder containing main.js router and users.js user details file)

----- main.js

----- users.js

----- orders.js

Here my users module is in the same folder as my router (main.js)

My code for router is:

var url = require('url');
var users = require('./router/users.js');

module.exports = function (app) {

    app.get('/', function (req, res) {
        res.render('index.html');
        console.log("Home page displayed");
    });
app.get('/login', function (req, res) {
        res.render('login.html');
        console.log("Log in page displayed");

    });

    app.post('/login', function (req, res) {
        var url_parts = url.parse(req.url, true);
        var query = url_parts.query;

    console.log(query);
    console.log("Username: " + query.useremail);
    console.log("Password: " + query.password);

    validUser = false;

    for (var x in USERS.users){
        //console.log(USERS.users[x].userid);
        if (query.useremail === USERS.users[x].useremail
           && query.password === USERS.users[x].password)              
        {
                validUser = true;
                break;
         } // end of if

    }// end of for

  console.log(validUser);

    if (validUser === true){
    console.log("logged in");
    res.render('index.html');
    }     

});
app.get('/api/orders/:id', function (req, res) {
      console.log(req.params.id); 
      res.json(ORDER.orders[req.params.id]);
    });

    app.get('/api/orders/', function (req, res) {
      console.log(ORDER); 
      res.json(ORDER);

    });    
 app.get('/api/users/:id', function (req, res) {
      console.log(req.params.id); 
      res.json(USERS.users[req.params.id]);
    });

    app.get('/api/users/', function (req, res) {
      console.log(USERS); 
      res.json(USERS);

    }); 

My code for users.js:

module.exports = function () {

// Create User prototype and objects
var USERS = { users: [] };

function User(type, useremail, password) {
 this.type = type;
 this.useremail = useremail;
 this.password = password;
}

var Bob = new User("rep", "bob@bob.com", "qwerty");
USERS.users.push(Bob);

var Helen = new User("rep", "helen@helen.com", "test");
USERS.users.push(Helen);

var Dominic = new User("customer", "dom@dom.com", "1234");
USERS.users.push(Dominic);

var James = new User("grower", "james@james.com", "pass1");
USERS.users.push(James);

};

I'm pretty new to this are but have been reading up alot on modules but still can't figure it out. Any suggestions on where I have gone wrong? or what I need to do to rectify the problem?

Note: Previously I did the same thing for including router into the server file using module.exports = function (app) { around my router and in my server as: require('./router/main')(app);