On Location Photography Help for 75 people group and individual Session?

I can't help with the individual shots, but I have done several groups. Here are two different years of a group with approx. 100 people in each shot. Read the exif date in the caption, including the lighting information. I used Nikon Speedlights because that's what I have. I actually got them with the first shot in mind, as I don't want to deal with massive banks of lighting outdoors.

http://www.flickr.com/photos/samfeinstein/49749100...

http://www.flickr.com/photos/samfeinstein/39347280...

A very important piece of equipment for group shots is a whistle! Let them know that the whistle is a one second warning. "Okay, everyone look at me. If you can't see me, I can't see you. Here is your warning...... [blow the whistle, pause a second, shoot]" Stick to that pattern so they get the rhythm right. Take 4 or 5 group shots so you will be more likely to get one where everyone is looking at you. I have had to lift a face or two from one shot to replace them in another in the past. Try to avoid shooting with the sun straight overhead. Watch out for extreme shadows on the face. I had no choice about time OR location of the group the first time I did it as there was construction on the site that made an ugly background, but everyone ended up with dark spots under their noses! I went through the entire picture and tap-tap-tap dodged out the darkest bit of the shadows. Otherwise, they all looked lie little Hitler mustaches and that just would not be suitable...

I think you could just write down the name of each subject and record the file number (such as DSC_4172) by each name. If you are afraid, you can write a note (male, blue stripe shirt) every now and then just to be sure you have your list in sync with the files. I am sure you would be able to devise lighting with 3 speedlights and a backdrop for the portraits also. Someone else will have a good recommendation for that, I am sure.