All of the photos you see in this post were shot using this technique with my 16-35mm 2.8 lens and holding the camera high above my head. I shoot 75% of the reception this way--not even looking through the viewfinder.
Before I share my focus trick, if you aren't familiar with back-button focus, click through the link to familiarize yourself with it. I shoot everything using the back-button to focus and nothing else I say here will make sense unless you understand what it is first.
I always shoot in auto-focus. But in low-light situations the camera struggles to focus. This is because the camera is looking for points of contrast in the subject in order to know what you want to focus on. In low light situations, there isn't enough light to create the needed contrast. So instead of going blind and spending the entire reception resisting the urge to slam my camera into the ground, this is what I do. I find a point of contrast somewhere in the vicinity. A candle in a centerpiece works great; a light of some type--anything you CAN focus on. I stand about 4-5 feet away from it and lock in my focus on it using the back button. Then I don't touch the back-button again for the rest of the night (or until I want to change my focus distance). I set my aperture to 4.5 (or higher) and then I shoot away with my camera above my head always standing about 4-5 feet away from my subject. With my aperture set higher (creating a greater depth-of-field) and my focus locked on this approximate distance, I can trust that everything I want in focus IS in focus. That's it. It's that simple.
The results are as follows:
And, instead of slamming my camera on the ground, I spend most of the night looking like a guest: dancing around the floor with my arms waving above my head having a jolly good time. Ok, maybe a bit goofier-looking than the guests. But it gets the job done.