code: func (b *Bslack) JoinChannel(channel config.ChannelInfo) error {
  	// We can only join a channel through the Slack API.
  	if b.sc == nil {
  		return nil
  	}
  
  	b.channels.populateChannels(false)
  
  	channelInfo, err := b.channels.getChannel(channel.Name)
  	if err != nil {
  		return fmt.Errorf("could not join channel: %#v", err)
  	}
  
  	if strings.HasPrefix(channel.Name, "ID:") {
  		b.useChannelID = true
  		channel.Name = channelInfo.Name
  	}
  
  	if !channelInfo.IsMember {
  		return fmt.Errorf("slack integration that matterbridge is using is not member of channel '%s', please add it manually", channelInfo.Name)
  	}
  	return nil
  }
docs: // JoinChannel only acts as a verification method that checks whether Matterbridge's
  // Slack integration is already member of the channel. This is because Slack does not
  // allow apps or bots to join channels themselves and they need to be invited
  // manually by a user.