Alright, I'll give you a simple example of how to set up user signup in a MERN (MongoDB, Express.js, React.js, Node.js) stack app. In this example, we'll make an easy registration form on the frontend and manage the registration process on the backend.

Backend (Node.js/Express):

1. Update the server.js file in the e-commerce-server directory:

const express = require('express');
const mongoose = require('mongoose');
const cors = require('cors');
const bcrypt = require('bcrypt');

const app = express();
const PORT = process.env.PORT || 5000;

app.use(cors());
app.use(express.json());

mongoose.connect('mongodb://localhost:27017/e-commerce', {
  useNewUrlParser: true,
  useUnifiedTopology: true,
});

const userSchema = new mongoose.Schema({
  username: String,
  password: String,
});

const User = mongoose.model('User', userSchema);

app.post('/api/register', async (req, res) => {
  try {
    const { username, password } = req.body;

    // Check if username already exists
    const existingUser = await User.findOne({ username });
    if (existingUser) {
      return res.status(400).json({ error: 'Username already taken' });
    }

    // Hash the password
    const hashedPassword = await bcrypt.hash(password, 10);

    // Create a new user
    const newUser = new User({ username, password: hashedPassword });
    await newUser.save();

    res.status(201).json({ message: 'User registered successfully' });
  } catch (error) {
    res.status(500).json({ error: 'Internal Server Error' });
  }
});

app.listen(PORT, () => {
  console.log(`Server is running on port ${PORT}`);
});

This backend code creates an easy registration endpoint (/api/register). It checks if the username is already in use, secures the password, and saves the new user to the database.

Frontend (React):

1. Update the src/App.js file in the e-commerce-client directory:

import React, { useState } from 'react';

const App = () => {
  const [username, setUsername] = useState('');
  const [password, setPassword] = useState('');

  const handleRegister = () => {
    fetch('/api/register', {
      method: 'POST',
      headers: {
        'Content-Type': 'application/json',
      },
      body: JSON.stringify({ username, password }),
    })
      .then((res) => res.json())
      .then((data) => alert(data.message))
      .catch((error) => console.error('Error registering:', error));
  };

  return (
    <div>
      <h1>User Registration</h1>
      <label>
        Username:
        <input
          type="text"
          value={username}
          onChange={(e) => setUsername(e.target.value)}
        />
      </label>
      <br />
      <label>
        Password:
        <input
          type="password"
          value={password}
          onChange={(e) => setPassword(e.target.value)}
        />
      </label>
      <br />
      <button onClick={handleRegister}>Register</button>
    </div>
  );
};

export default App;

The frontend code has a simple registration form with spaces for a username and password. When the user clicks the "Register" button, their details are sent to the backend (/api/register).

Be sure to run your MongoDB server and then start both the frontend and backend applications:

# In the e-commerce-server directory
node server.js

# In the e-commerce-client directory
npm start

This is a simple example. In a real-world app, you'd want to add features like password confirmation, form validation, and better error handling. Also, think about using HTTPS in a production setting for secure communication.