In it’s most basic form you don’t need to make it be a group, that was was specif to one problem and might be more than necessary for you.
If the object you want to cut is raw geometry simply place the cutting plane though it, select all of it, right click and Intersect with Selection.
It’s possible that you have downloaded a Group and made it a group again, causing it to become a nested group …
So as mentioned above the model would help, at the very least a screen shot.