Noticing that all the front tables had someone either sitting at them or having reserved a seat, I went to the back and found several chairs facing the large window at the wall. I moved the two remaining empty ones together, put my writing pad on one of them and went back to the front for my jacket to put on the other.
By the time I returned to the back, jacket in hand, a woman was pulling one of the chairs aside. I asked, in a soft voice, "Would you like me to get you another chair? I'm using these two." She declined in a nice tone of voice, and mentioned that she'd figured that possibly both the chairs were reserved. As she was walking away, I repeated my offer, which she again refused and then she left.
By (1) addressing the situation indirectly, (2) offering a favor, (3) using a soft voice and (4) repeating my offer even after it was clear I would get what I wanted, I helped her feel much better about the situation. Had I just said, in a normal or even a strong tone of voice, "I'm sorry, I'm using both chairs - you'll need to get another one," maybe she would have still left but she would not likely have been happy about it.
And on the other hand maybe she would have put up a fight - with a best-case scenario a bit of unpleasantness and serious social fuel expenditure right before the meeting, medium-case also having to find another chair and worst-case an embarrassing scene in front of the applicant (who arrived a short time later).
Everyone happy, and a great meeting was had by all.