https://docs.microsoft.com/en-us/bot-framework/nodejs/bot-builder-nodejs-handle-conversation-events

bot.on('conversationUpdate', function (message) {
    if (message.membersAdded && message.membersAdded.length > 0) {
        // Say hello
        var isGroup = message.address.conversation.isGroup;
        var txt = isGroup ? "Hello everyone!" : "Hello...";
        var reply = new builder.Message()
                .address(message.address)
                .text(txt);
        bot.send(reply);
    } else if (message.membersRemoved) {
        // See if bot was removed
        var botId = message.address.bot.id;
        for (var i = 0; i < message.membersRemoved.length; i++) {
            if (message.membersRemoved[i].id === botId) {
                // Say goodbye
                var reply = new builder.Message()
                        .address(message.address)
                        .text("Goodbye");
                bot.send(reply);
                break;
            }
        }
    }
});

event去年也沒有，但那個時候line就有了，我記得我自已硬加的，不然很難搞，不過現在有這些定義好的event可以接，就方便很多了。