[kiterunner]

An idea I had today. If you have a DNA match on ancestry who has a private tree, and there are "shared matches" whose trees you can view, try searching your match list for each surname from those other trees and see if the person with the private tree comes up for any of them. Since the surname search does kindly let you know if you have guessed a surname in a private tree! This might give you some clues as to how the group of shared matches are connected to each other and to you. It worked for me.

(I'm assuming you have already searched your match list for the surnames from your own tree.)