This will ping every single user on your Slack team every time they go active with "u up?"

uup.rb

require 'slack-ruby-bot'

class UUpBot < SlackRubyBot::Bot
  command 'yeah' do |client, data, match|
    client.say(text: 'me too', channel: data.channel)
  end
end

class UUpServer < SlackRubyBot::Server
  on 'presence_change' do |client, message|
    username = client.users[message.user].name
    if message.presence == 'active' && username != client.name
      client.web_client.chat_postMessage(text: 'u up?', channel: "@" + username, as_user: true)
    end
  end
end

UUpBot.run
run UUpServer.new