There was a lot of deviation on this thread as people misunderstood or listed other situations and then others assumed that to be the case. Now that you've clarified:
You arrived at a lean-to that did not contain one large camping area but instead included three separate designated tentsites (pads or platforms) and all three were occupied in some fashion. So the awake couple weren't taking up the entire camping area - they were in one specific site?
I have to admit that in places that have already been subdivided into specific sites, I do think of those sites as per-group. I wouldn't encroach onto another's pad or platform unless directed to by a caretaker or if it was posted that one site was specifically for sharing or "larger groups."
You arrived at a lean-to that did not contain one large camping area but instead included three separate designated tentsites (pads or platforms) and all three were occupied in some fashion. So the awake couple weren't taking up the entire camping area - they were in one specific site?
I have to admit that in places that have already been subdivided into specific sites, I do think of those sites as per-group. I wouldn't encroach onto another's pad or platform unless directed to by a caretaker or if it was posted that one site was specifically for sharing or "larger groups."