When your at a wedding or an event sometimes you want to get a photo of everybody who's there. When photographing a large group of people it's important to elevate yourself a little bit, look for a staircase or look for a balcony that you can perhaps work off. I generally try not to work off a balcony just cause it gives me too much height and I'm looking down at subjects who are looking up and it creates a pretty un-artful photograph.
But, you know, three or four stairs up or a step ladder if you're taking one for a proper shoot. You'll notice most of the rule of thirds is gone out, I've got my wedding couple dead center. The only rule of third I'm using here is my top horizon as being the positive space and the negative space. It's also important to try not to let guests be silly and put arms up, you know, blocking other guests. But sometimes, after a few drinks, things are going to happen. It's also really important to have a bit more of a wider aperture. F8 is perfect for shooting a large group of people because your maintaining that focus throughout the crowd. Lighting is probably one of the key things, try and find a well lit room. You'll probably want to bounce a speed light off the ceiling at a fairly high intensity.
And you wanna try and fill as much of the space as possible. ISO 800 is also going to help with this. When shooting a large group of people, just remember, try and fit everyone in the frame.