join_all_slack_channel.rb

require 'slack'

Slack.configure do |conf|
  conf.token = ENV.fetch('SLACK_TOKEN')
end

client = Slack::Web::Client.new
channel_names = client.channels_list(exclude_archived: true).channels.reject(&:is_member).map(&:name)
channel_names.each do |name|
  puts "Join to #{name}"
  client.channels_join(name: name)
end

Join all public channels.
It does not support shared channels.