Hi Samuel,
I think before you get into the technical aspect, camera settings, flash power, lighting position, you need to take a step back and set some expectations with yourself and with the bride and groom. It's a slow work day so I'll try to help you best I can, feel free to PM me if you want to take this off the general board. Ok, so first I would advise that you take a deep breath and realize that the assignment you have volunteered for is one of the most difficult photographic aspects at any wedding. I will describe my typical reception strategy and thought process. From that I think you need to distill it down to a subset and be comfortable with that subset. If you try to do too much without the right experience or practice, it'll all go to hell.
A typical wedding reception includes the following types of pictures:
1. Group pictures 2 - 8 people dancing/partying
2. Bride/Groom/Daughter/Father/Mother/Son dancing
3. Cake cutting
4. Candids
5. Speeches/bouquet toss/floor events
6. details (food, cake, arrangements)
From people sitting down to eat until they are sloshed and dancing like a fiend, you're gonna run that gamut. In order to capture all those scenarios in a dark room, I employ the following strategies:
0. Generally lighting is provided by two off camera flashes on light stands that is triggered by an on camera trigger or strobe. This provides background lighting, accent lighting, hair lighting, and sometimes direct lighting when I want shadows.
1. Group pictures from 3ft - 10ft away are handled via on camera flash with bounce card set to TTL and adjusted as necessary. TTL will be thrown off constantly because of black suits and dresses. More advance flash users will know that x distance requires x power. I typically start at f5.6 ISO 800, 1/8th power. Take a pic, check it and then dial it up or down as needed. I know this is effective between 3ft and 8ft. Any further and I need to dial power up, any closer and i need to dial it down. Don't off camera flash it with flash in hand even if you have good experience. The results won't be THAT much better and you'll be holding your camera with one hand and flash in the other. 1. you'll have no stability. 2. it looks awkward 3. it looks awkward. Gary Fong Sphere does not work between 3 ft, 5 people, and 10ft ceilings.
2. Dancing pictures are done the same way. You want to freeze action but you will typically be far away. Nothing wrong with a little motion blur when dancing. Two camera bodies and two lens, one long, one wide. When taking wide shots, you take a test shot and decide how much ambient you want. Start with shutter and set it to 1/125th for example, aperture can be wide open if there are two people, I typically go f4, then set my ISO until I get the desire lighting level for the ambient. Now, if the place is bright, maybe my that's good enough. But if not, I then dial my on camera flash with bounce card until my couple is lit. When shooting long for tight shots, I go same strategy but a higher shutter to avoid camera shake. 1/200th, f4, iso to ambient flash to fill.
3. Cake cutting, same as 1.
4. Candids, same as 1, or 2 with the long lens
5. Same as 2.
So as you can see, regardless of the situation the strategy is the same, you have to have a wide game, and a long game. Start with shutter for motion, aperture for DOF, iso to whatever matches ambient, flash to fill. Those are the basics. You have a 5DM3 so don't be afraid to push that sucker all the way to ISO 6400 or beyond.
Ok, so that's the strategy and you're not a wedding photographer so that means maybe you don't have two camera bodies, wide and long lens, off camera flashes with triggers. But you do so you can technically do everything I said above except for the off camera part. Whether you want to comes down to whether you want to keep things simple or not.
At the most basic you have a 5DM3, flash, and normal lens. So you're good to go on
Group shots, wide dancing shots, cake cutting, and candids. Keep it simple, on camera flash and bounce card and you'll get 85 percent. Things you wont get: long tight cropped shots. Good background lighting. But as long as you're ok with that then that's the way to go.
You've never shot a wedding reception so your stuff isn't going to be amazing but keep to what you know. Set expectations with the bride and groom. Tell them that the picture of Uncle George giving a speech is going to be wide and Uncle George will be lit with flash but everyone in the background will be dark. That's ok.
and hey, if you have good glass then shoot candid stuff wide open. I always do.
Things to remember:
1. don't Try to do too much
2. don't try to achieve a particular style or look that you have no experience with
3. don't Beat yourself up. Just cause you're a great photographer doesn't mean you're a great wedding photographer.
4. don't Stress too much about the technical aspects. Remember the basics. There is no magic combination that always works. But if you make it so that you only have one adjustment point (flash power), then it's easy to adjust.
5. don't shoot groups wide open. People always stagger and some will be out of focus. You don't need f8, f5.6 - 6.3 is usually good for at least 4ft deep. If you don't know this, then you need to read up on how dof works.
6. You don't need off camera lighting at a wedding reception unless it's accent, background, and achieving a particular style.
7. don't call yourself a natural light photographer. In photography circles it just means you never learned how to use a flash. It is not possible to shoot in the dark when you are a natural light photographer and tons of great photography work is done in the dark.
I'm going to say this one more time just in case you forgot. Set expectations with yourself and the bride and groom so there is no confusion when there are no brightly little first dance pictures. They need to know what they are getting and you need to know your limitations. Hope this helps. Sorry, a little preachy.