get emoji reactions for a message in Slack – unfortunately their API "might not always contain all users that have reacted". limited to 50 as of 2/12/16.

slack_reactions.js

'use strict';

const Promise = require('bluebird');
const WebClient = require('slack-client').WebClient;

const token = process.env.SLACK_API_TOKEN;
if (!token) {
  throw "Please set SLACK_API_TOKEN";
}

const client = new WebClient(token, {logLevel: 'debug'});


const listChannels = Promise.promisify(client.channels.list, {context: client.channels});
const getReactions = Promise.promisify(client.reactions.get, {context: client.reactions});
const listUsers = Promise.promisify(client.users.list, {context: client.users});


// returns a Promise for the channel object
const getChannel = (name) => {
  return listChannels(true).then((channels) => {
    return channels.channels.find((channel) => {
      return channel.name === 'news';
    });
  });
};

// returns a Promise for the list of reaction objects
const getReactionsFor = (timestamp, channelId) => {
  const opts = {
    channel: channelId,
    timestamp: timestamp
  };

  return getReactions(opts).then((data) => {
    return data.message.reactions;
  });
};

const getUsersById = () => {
  return listUsers(false).then((data) => {
    let usersById = {};
    data.members.forEach((user) => {
      usersById[user.id] = user;
    });
    return usersById;
  });
};

const printReactions = (timestamp, channelId) => {
  Promise.all([
    getReactionsFor(timestamp, channelId),
    getUsersById()
  ]).then((results) => {
    const reactions = results[0];
    const usersById = results[1];

    reactions.forEach((reaction) => {
      console.log(`${reaction.name}:`);
      reaction.users.forEach((userId) => {
        const user = usersById[userId];
        console.log(`  ${user.profile.email}`);
      });
    });
  });
};


getChannel('news').then((channel) => {
  printReactions('1454530550.000057', channel.id);
});

Here's a rewrite with the latest Slack API for anyone curious:
https://gist.github.com/ahallora/b40684c8d705147bf8a790c44b82a22f