join.rb

require 'slack-ruby-client'
slack = Slack::Web::Client.new(token: ENV['SLACK_API_TOKEN'])

members = []
next_cursor = nil

loop do
  slack_users = slack.users_list({ limit: 1000, cursor: next_cursor })
  members << slack_users['members']
  next_cursor = slack_users['response_metadata']['next_cursor']
  break if next_cursor.empty?
end
members.flatten!

channels = []
next_cursor = nil
loop do
  slack_channels = slack.conversations_list({ limit: 1000, cursor: next_cursor })
  channels << slack_channels['channels']
  next_cursor = slack_channels['response_metadata']['next_cursor']
  break if next_cursor.empty?
end
channels.flatten!

channels.each do |c|
  next_cursor = nil
  loop do
    c['members'] ||= []
    break if c['num_members'] == 0 || c['is_archived']

    channel_members = slack.conversations_members({ channel: c['id'], limit: 1000, cursor: next_cursor })
    c['members'] << channel_members['members']
    next_cursor = channel_members['response_metadata']['next_cursor']
    break if next_cursor.empty?
  end
end

user = members.find { |m| m['name'] == ENV['SLACK_USER'] }
exclude_channels = ENV['SLACK_EXCLUDE_CHANNELS'] ? ENV['SLACK_EXCLUDE_CHANNELS'].split(/,/) : []
channels.each do |c|
  next if c['is_archived']
  next if c['members'].empty? || c['members'].flatten.find { |m| m == user['id'] }
  next if exclude_channels.include?(c['name'])
  next if c['is_ext_shared']

  begin
    slack.conversations_invite({ channel: c['id'], users: user['id'] })
  rescue Slack::Web::Api::Errors::AlreadyInChannel
    next
  rescue Slack::Web::Api::Errors::NotInChannel
    slack.conversations_join({ channel: c['id'] })
    begin
      slack.conversations_invite({ channel: c['id'], users: user['id'] })
    rescue Slack::Web::Api::Errors::AlreadyInChannel
      next
    ensure
      slack.conversations_leave({ channel: c['id'] })
    end
  rescue StandardError => e
    pp c
    pp e
  end
end