I would make a duplicate of your group and attach all to one editable mesh with blur’s attachselection script. then apply the slice to that…nice and clean (slicing a group probably no fun)
Usually the blur script works really nice - in fact I use it ALL the time. But on really complex scenes with a lot of textures and geometry it tends to get the groups mixed up.
I tried several different methods without buying additional software but ended up just throwing in a plane and cutting/shifting everything manually.
Put as many objects as you can on a layer that you can simply hide. Then you can actually apply a slice modifier to several objects at once. It will be instanced between all the objects selected and will have the same transforms. Set it to Remove Top and rotate it into place.
Tip: Turn off the modifier before moving the slice gizmo so that you don’t have wait while all the objects update in the viewport.
You can also use the Align tool to position the Gizmo. so create a Point helper where you want the slice to occur. Rotate it so the XY plane is parallel to your section direction. This way if you have to add new slice mods, you can alighn their gizmo to the point object.