There are a few ways to handle a large group talking simultaneously, depending on the situation.
If the group is a constant background noise that is part of the setting, they can be written into the scene description:
INT. CONVENTION HALL – DAY
The massive room is packed full of Browncoats, BUZZING about all things Firefly.
Phil has to shout to be heard.
If the group’s timing is important, they can be handled like any other key sound:
MAYOR WILSON (CONT’D)
So vote for me, and we will have another four years of Hill Valley prosperity!
The crowd ERUPTS in support.
If the group’s message is particularly important, you can call attention to it by writing some of it out with slashes:
VARIOUS CROWD MEMBERS
He lies!/Don’t believe him!/We demand proof!/etc.