Umm, yeh. Horde difficulty is set when the horde starts, and remains at that level the entire night. If you're standing in Pine Forest at 22:00, you'll have an easy horde. Even if you then move into a base entirely in the wasteland; but nothing wrong in having one that spans both.
For groups, I think everyone needs to be in the Pine; the group difficulty is a weighted average, heavily favoring the highest individual GS, so anyone in the Wasteland will increase the stage dramatically.