I have a Canon 5D Mark III. I understand the depth of field issues taking group photographs at a wedding, but I am concerned the guests at the extremities might be less in focus than the centre if I use AI focus. I will be using a 24-70mm lens.
AI (Auto-Intelligent) Focus (It is an uppercase "i", not a lowercase "L" or a numeral "1") is one of those things that sounds good in theory but often doesn't work well in practice. Sometimes it does, but at other times it doesn't. I prefer to set my camera to either One Shot or AI Servo, rather than splitting the difference with AI Focus that starts in One Shot mode and then switches to AI Servo if it senses the subject distance has changed. Using the custom control menu in most Canon cameras makes it very easy to set a control button to function as a switch between One Shot and AI Servo. I normally have the DoF Preview button programmed to that function with my 5D Mark III so that I can switch from one to the other without even removing my eye from the viewfinder. I also sometimes set my cameras so that a shutter button half press does not activate AF at all. By using the AF-ON button as the only way to activate AF, you can leave the Camera in AI Servo mode and just stop pressing the AF-ON button to lock focus a la One Shot mode. In this scenario, however, it's not likely to make much of a difference since it is doubtful you'll be using the focus and recompose technique.
What your real concern should be is the performance of your lens at the edges of the photo compared to the performance of your lens at the center of the photo. If the center is razor sharp but the edges are a little soft it is more noticeable than if the entire image is uniformly a little soft! At wide open apertures most lenses are softer on the edges than at the center. Some more so than others. Also, if a lens has alignment issues it will normally show up more distinctly as blur on the edges of the frame.
Since you didn't specify exactly which lens you are using, we can't assume it is the EF 24-70mm f/2.8 L II which is very good from center to edge at apertures above f/4 and most focal lengths. The Tamron SP 24-70mm f/2.8 Di VC is also very good at f/4 and above at most focal lengths. The original Canon EF 24-70mm F/2.8 L is a little less so even at f/5.6. The latter is also subject to having problems associated with a de-centered front element as even moderate bumps to the front of that lens can cause optical alignment problems. This issue normally shows up to to greater degree at the edges than in the center of photos taken with a misaligned lens. I never go out in public with my vintage EF 24-70mm f/2.8 L without the protective hood in place. Due to the unique design of that lens where it extends at wider focal lengths and retracts at longer focal lengths, the hood is attached to the main barrel and protects the inner barrel as it extends and retracts. It is the front of the inner barrel that is affected by alignment issues caused by bumps to the front of that lens.
Since you are setting up portable lights to use for the formal group shots (you are doing this, aren't you?), it frees you up to use the best aperture for your particular lens without having to worry about exposure issues or using a high ISO. Just experiment (a flat brick wall works well for this) at different apertures and focal lengths while shooting from a similar distance as you will at a wedding to find where your lens gets the edges of the frame acceptably sharp as it does in the center. When shooting large groups that can't all fit in a single row, also remember to allow for deeper depth of field since some of the people in the scene will be closer or further from the camera than others.
Maybe invest some time in DoF Master online or use DoFViewer app to do some distancing ahead of time.
Maybe best not to go much below 35mm when shooting the group photo as rectilinear distortion creeps in markedly at the edges (bowing effect). It can be dealt with in LR - Develop - Lens adjustment but better to avoid in camera.
Take a "heads up" view of the lighting conditions. Take a test shot on P setting and spin the dial to see what combinations of parameters will work on the day. If within the envelope of settings chosen try and "foot zoom" to stand further back and aim to get aperture towards f/11 (at least as an ideal as this is not normally possible to achieve).
Maybe expose to the lighter area e.g. that wedding dress and if necessary set exposure compensation down a stop or two. You can normally bring back the detail in the shadows.
If off camera flash is needed then you're constrained to standing closer to your subjects. Don't forget if using flash you set two exposures, first is normal camera setting for background detail versus darkness and secondly the flash exposure. Talking the talk is fine but practice is the key.