Producing Assistant Crew/Group Conductor
The Assistant Director's Crew usually leads the Producing Assistant Crew. You'll need Production Assistants and Crew Chiefs to coordinate the crowd to keep them in line and to control and direct the crew. This includes preparing their costumes and make-up, and guiding them into formation as directed by the director - not to mention feeding and watering them, as well as any medical or physical issues that may arise with the actors.
Walkie Talkies/Headsets
On larger, more complex shoots, the most important tool is a walkie-talkie system with a solid signal. Dealing with a large crowd of actors is nearly impossible if you can't immediately communicate with the assistant director's crew and the assistant producer's crew. Be sure to have backup communication equipment, batteries and backup charging to ensure you don't lose contact.
Loudspeaker/megaphone
In the case of a large-scale group shoot, a tweeter and megaphone system is the only option to allow your team to direct the dispatch since large crowds tend to be noisy, scattered, and difficult to communicate with and direct. It's helpful when the crowd needs to react to cues or perform synchronized movements. Getting them all together for a brief "instant reaction" with a tweeter is your only option.