I've photohgraphed literally hundreds of sorority and fraternity groups. Did it for years before going into weddings. I know exactly what thery're asking for, so here's some tips.
1. Find a big shaded area, or at least someplace with low contrast light. Usually in front of behind the sorority house, depending on the sun's location.
2. 35mm, ideally at f/8. Crank up ISO if necessary, but don't use a long lens or the back row will be all blurry.
3. Bring a setp ladder. It's nice to get 2-3 steps higher than the first row of ladies so you're shooting down a little bit. You'll see the back rows better this way.
4. Tripod.
5. Set camera to auto-bracket
6. Sorority ladies are EXPERTS at posing Seriously. I'm not knocking them at all, but they pose for group photos literally all the time. You will be amazed when they set themselves up nearly perfectly, and all you need to do is make minor adjustments. Typically, they pose themselves new members kneeling in front and older member in back.
7. They will want to do a fun pic at the end. Yell out "fun shot," and they will think you do this everyday.
Most importantly...stand far enough back to leave 10% of the viewfinder empty on both sides of the group. This way you can crop to 8x10s without cutting anyone off the side.
One of the companies that does a lot of these is GreekYearbook.com