In addition to Hal's comments, I might add some thoughts from a guerrilla group shooter. Depending on the group, you can build risers out of folding chairs, which might be possible at the reception. Try to keep your rows from 9 to 15 folks wide.
With 65 try a row of 16 chairs facing away from the camera that agile folks can step up onto. Have 15 folks stand in choir windows in front of them. Add 16 chairs in front of them and have the less agile sit on them. Finally have the most agile sit, squat, or kneel in front of them.
When my groups get large, I remove all the modifiers. I still get good results.
Scott Chapin Powder Springs, GA, USA Nikonians Team Member