Welcome to the forum.
Yes, there are some of methods that can work to help people look slimmer in photos...many of which you have listed already. A very important one that you didn't list was to use short lighting, rather than broad lighting. In other words, when you have some sort of side lighting, have them turn toward the light so that you are lighitng their face and their far ear (away from the camera). That is short lighting.
The opposite is broad lighting, where they are turned away from the light and you are lighting their near ear and basically their whole head...thus making it look larger in the photo.
When taking a group photo, it can be much harder to establish nice portrait lighting on all the members of the group...so it may be very hard to achieve short lighting on all of them...but at least try to avoid an obvious broad lighting situation.
A basic technique is just making sure that you avoid posing the people with their shoulders square to the camera. This gives them the maximum area/girth in the photo, so turn their shoulders/bodies. Of course, watch for things that will hurt your cause...like big bellies showing up in profile or lines appearing on their necks as they look back from their turn.
As you may know from shooting senior portraits, the clothing & background choices will make a huge impact on the outcome of the photo. You usually want to key the clothing to the background, so that the faces are where the viewer's eye will be attracted. So if your subjects have large bodies, you can draw attention away from that by having their clothing blend into the background, rather than contrasting with it.
Lastly, it's very likely that large people know that they are large. If you go too far in trying to make them look thin, it may not capture the real them...and it may even make them feel self conscious, which can translate into bad photos. I think that it's much more important to make them feel comfortable and let their personalities show.