BotFrameworkのMembersAddedを使うと、Slackのチャンネルに新しく入ってきた人に挨拶できるみたいなので、少しやってみました(^O^)v
※MembersAddedのドキュメントはこちら

入ってきた人に挨拶

var restify = require('restify');
var builder = require('botbuilder');

var server = restify.createServer();
server.listen(process.env.port || process.env.PORT || 3978, function () {
   console.log('%s listening to %s', server.name, server.url); 
});
  
var connector = new builder.ChatConnector({
    appId: 'ID',
    appPassword: 'PASSWORD'
});
var bot = new builder.UniversalBot(connector);
server.post('/api/messages', connector.listen());

// ここで入ってきた事を取得して、挨拶
bot.on('conversationUpdate', function (message) {
    if (message.membersAdded) {
        var membersAdded = message.membersAdded
            .map((m) => {
                var isSelf = m.id === message.address.bot.id;
                return (isSelf ? message.address.bot.name : m.name);
            })
            .join(', ');

        var reply = new builder.Message()
            .address(message.address)
            .text('いらっしゃいませー ' + membersAdded + ' さん');
        bot.send(reply);
    }
});

退出した時に挨拶

退出時はmembersRemovedを使うと、取れるみたいです。

  if (message.membersRemoved) {
        var membersRemoved = message.membersRemoved
            .map((m) => {
                var isSelf = m.id === message.address.bot.id;
                return (isSelf ? message.address.bot.name : m.name);
            })
            .join(', ');

        var reply = new builder.Message()
            .address(message.address)
            .text('ばいばーい' + membersRemoved);
        bot.send(reply);
  }