How do you set a rally?

Select a target to attack. Either player enemy, or Wendell / Voracious Zombies. For the Player enemies there will be an option for 'Rally' as well as a simple attack option. For the Boss zombies there is only a rally option.

Choose Rally and set the time for the rally to start (up to 60 minutes) to allow others to join. And then select your APC to lead the rally.

For the boss zombies you need to have at least 2 additional rally slots available for others to join otherwise it will fail. Upgrade your Headquarters building to increase the slots.