code: func (c *cmd) startupJoin(agent *agent.Agent, cfg *config.RuntimeConfig) error {
  	if len(cfg.StartJoinAddrsLAN) == 0 {
  		return nil
  	}
  
  	c.UI.Output("Joining cluster...")
  	n, err := agent.JoinLAN(cfg.StartJoinAddrsLAN)
  	if err != nil {
  		return err
  	}
  
  	c.UI.Info(fmt.Sprintf("Join completed. Synced with %d initial agents", n))
  	return nil
  }
docs: // startupJoin is invoked to handle any joins specified to take place at start time