This must be an easy one, but I can only think of specialized alternatives at the moment: what do you call someone you are about to have a meeting with? Meetee?
In more specific contexts client, friend, date would all work, but they convey more than the fact that you will just be meeting. I've heard appointment used in that sense (as in "your next appointment is waiting outside", but the ambiguity (appointment being a lose synonym for meeting) is not really satisfactory.
Ideas?