Install ghost from source

First, you should know how to develop with ghost.

create a new react app under core/server folder

simple example

core/server$ mkdir newapp
core/server$ cd newapp
core/server/newapp$ 
Ghost/core/server/web/newapp$ ls
app.js  controller.js  index.js
Ghost/core/server/web/newapp$ cat index.js 
module.exports = require('./app');
Ghost/core/server/web/newapp$ cat app.js 
const express = require('express');
module.exports = function setupNewApp() {
    const newApp = express();
    newApp.get('*', require('./controller'));
    return newApp;
};
Ghost/core/server/web/newapp$ cat controller.js 
const path = require('path');

module.exports = function newAppController(req, res) {
    res.send('Welcome to new App!');
};

Add route for your new site/app

./core/server/app.js

    // Mount the  apps on the parentApp
  
    // ADMIN
    parentApp.use('/ghost', require('./admin')());

    // import new app/site
    parentApp.use('/newsite', require('./newapp')());
    
    // BLOG
    parentApp.use(require('./site')()); 

Notes

above are for ghost before 2.0 version.
for latest ghost, version 2.0,
The folder for new app should be "Ghost/core/server/web/newapp"
The hacked core js is Ghost/core/server/web/parent-app.js
The code for add route is same as above.
Please be sure that your new route is add before blog's route.
Otherwise, you will got 404 error.

Work your new app

grunt dev
navigate to http://localhost:2368/newapp, you will get: