You need to learn how to set your camera, which aperture to use, how to figure out what will work in various settings - before you get out there with a group of people trying to do a photo shoot. It might have been better to refer the friend to a professional or someone who knows how to do this. If you want to learn, get out somewhere that could be a potential location for future shoots and take pictures (of the trees/the scenery) and figure out what worked or what didn't.
This depends on how sunny it is that day. As Designer said, it depends on how you arrange them, you might need a shorter lens. You might do fine with f8, or you might need f11 - we can't tell you for sure without being there. That's why you need to do more learning, no one can tell you exactly how you'll need to set the camera without being there to meter the scene and figure out lining them all up and depth of field, etc. You probably will need to go to the location and do some test shots to figure out what to do.