To create a conversation table in MongoDB for Node.js and Socket.io, you can follow these steps:

1. Define a schema for conversations. This schema will define the structure of each document in the conversation collection.

2. Create a MongoDB collection to store the conversations. You can use the MongoClient object to connect to your MongoDB database and create the collection.

3. Define functions for adding and retrieving conversations. These functions will use the MongoDB driver for Node.js to interact with the conversation collection.

4. Use Socket.io to handle chat events, such as sending messages and joining or leaving conversations.

Here's an example code snippet that demonstrates these steps:

// 1. Define a schema for conversations
const mongoose = require('mongoose');

const conversationSchema = new mongoose.Schema({
  participants: [{ type: mongoose.Schema.Types.ObjectId, ref: 'User' }],
  messages: [{
    sender: { type: mongoose.Schema.Types.ObjectId, ref: 'User' },
    content: String,
    timestamp: { type: Date, default: Date.now }
  }]
});

// 2. Create a MongoDB collection to store the conversations
const MongoClient = require('mongodb').MongoClient;
const url = 'mongodb://localhost:27017';
const dbName = 'chat-app';

MongoClient.connect(url, function(err, client) {
  console.log("Connected successfully to server");
  const db = client.db(dbName);
  db.createCollection('conversations', function(err, res) {
    if (err) throw err;
    console.log("Conversation collection created!");
    client.close();
  });
});

// 3. Define functions for adding and retrieving conversations
const Conversation = mongoose.model('Conversation', conversationSchema);

function getConversationById(conversationId, callback) {
  Conversation.findById(conversationId).populate('participants', 'username').exec(callback);
}

function getConversationsForUser(userId, callback) {
  Conversation.find({ participants: userId }).populate('participants', 'username').exec(callback);
}

function createConversation(participants, callback) {
  const newConversation = new Conversation({
    participants: participants,
    messages: []
  });
  newConversation.save(callback);
}

function addMessageToConversation(conversationId, senderId, messageContent, callback) {
  Conversation.findByIdAndUpdate(conversationId, { $push: { messages: { sender: senderId, content: messageContent } } }, { new: true }, callback);
}

// 4. Use Socket.io to handle chat events
const io = require('socket.io')(http);

io.on('connection', function(socket) {
  console.log('A user connected');

  socket.on('join conversation', function(conversationId) {
    socket.join(conversationId);
    getConversationById(conversationId, function(err, conversation) {
      if (err) throw err;
      io.to(conversationId).emit('conversation data', conversation);
    });
  });

  socket.on('send message', function(data) {
    addMessageToConversation(data.conversationId, data.senderId, data.messageContent, function(err, conversation) {
      if (err) throw err;
      io.to(data.conversationId).emit('conversation data', conversation);
    });
  });

  socket.on('disconnect', function() {
    console.log('A user disconnected');
  });
});

Again, this is just an example, and you may need to adjust the code to fit your specific requirements.