//Bot on
bot.on('contactRelationUpdate', function (message) {
    if (message.action === 'add') {
        var name = message.user ? message.user.name : null;
        var reply = new builder.Message()
                .address(message.address)
                .text("Hello %s... Thanks for adding me. Say 'hello' to see some great demos.", name || 'there');
        bot.send(reply);
    } else {
        // delete their data
    }
});

More Info: https://docs.botframework.com/en-us/node/builder/chat-reference/modules/_botbuilder_d_.html