Depth of field shouldn't be an issue if you are far enough back to fit groups of 15-20 people in the frame and have plenty of daylight. Anything over f/4 with about a 50mm lens should be good.

This photo was shot using a full frame camera and 50mm lens at f/3.5 from a ladder about 1/3 of the way across a dimly lit basketball court (and then cropped slightly). I made the mistake of focusing on the front instead of middle row, but the DoF for anything but a gigantic print is still acceptable. If I had focused on the middle row as I should have even f/3.5 could have been printed up to about 50x32 inches. Although some of the portraits from a wedding may need to be displayed that large, I doubt any of the group shots would need to be.

This larger group of about 60 was also shot with a full frame camera and a 50mm lens at f/3.5 from further back (I was at mid-court and bounced a single flash connected by an off shoe cable off of the opposite wall behind me). The DoF doesn't hold up quite as well at larger sizes, but was perfectly acceptable printed at 8x10 in a high school athletic program. Oh, and the shutter speed was 1/50 second.

In my experience getting large groups to all pose correctly for the same instant is more about how you approach them and communicate that you need ONE good pose. If you take 15-20 shots each person will think it is OK if they have their eyes closed in just 2-3 of them. But the laws of probability say if each of 60 persons are not right in 2-3 of 15 pictures then the statistical odds say that on average 12 of the 60 will be blinking in any one shot and the odds that you will have even one photo with everyone correct is pretty much zero. You'll never get a group that large with everyone perfect, but I find I get a better results with fewer bad faces when I emphasize that this is THE shot (and then cheat a little by shooting a 3 shot burst). All but one of the frowners in the large group above are the drummers that think they are too cool to ever smile for a photo.

On a more general note my biggest piece of advice for large groups is to shoot from an elevated position. If there isn't a natural spot with the light at a good angle for that time of day, then use a ladder.

Also remember that most depth of field calculators are based on the assumption of an 8x10 print viewed at 10 inches by a person with 20/20 vision. For a larger display size viewed at the same distance you will need a narrower aperture to get the same depth of field.