Organizing BIG crowd of extras. HELP !

Why are they showing up at different times? Is that what you want?

On set, make sure your 2nd AD and a few PAs are in constant control of the crowd. You should be as focused as possible on the shoot itself, crowd control can't be your primary concern.

As far as communication, set up an e-mail group, or a twitter account for everyone to follow, or a mass-text group, or a facebook page, or something. Any way that you can communicate quickly and easily with the whole group.

Setup a single point of entry. No one gets in without signing a release form.

There should be a separate holding room for all extras to report to and wait in, until you need them on set. Make sure there's enough chairs, and a separate crafty table for them to snack on. have a PA in there to give them directions to the bathroom and answer any general questions they have.

Professional extra casting agencies use either mass emails or, more commonly, a voicemail line. You record your voicemail service with a message that explains the necessary wardrobe requirements, location / directions info, and call time. Usually calltimes are constantly changing, so let them know to check the line often as you will be updating the message when there are changes.

If you don't have the time / dedicated phone number for a voicemail line, stick to mass email. Add all the extras' email addresses to one BCC group and send them updates any time there are call time changes.

Once they arrive to set, have them all sign release forms and/or w2s if necessary. On larger professional productions the director will never even have contact with the extras, but if they are friends of yours make sure they know your are very appreciative that they could come and help out.

I would suggest organizing some events. Or maybe visit some local clubs. What age group do you want?
If college level - you could ask your local university to send out messages to the students .

Thanks a lot guys, so many useful tips were given... helps A LOT with the organization and knowing how to control things once on set. The idea of getting the university to send out a message sounds like a great technique!
Anyone know of a good release form template found online I could download?
As for people getting there at separate times, its not necessarily what I'm planning but I'm sure not everyone will get there at the same time... There is no compensation for the extras... and in my experience, when a crowd of people is not getting paid, they tend to be late.
Thanks again...
super helpful