A shield exists both as a barrier around the entire person and as a single point along with everything in between. (In a way, this is like the Bore, which does not actually exist as Shayol Ghul. The Bore exists everywhere, but Shayol Ghul is the place where it can best be detected. Which is not to say that there is any connection between the Bore and a shield. Both simply exist in different states simultaneously.) Someone who is shielded and trying to get past the shield can "feel" their way along its inner "surface" hunting for weaknesses, such as the points that indicate where the shield is being maintained or has been tied off. Shielding against the One Power will indeed stop someone from reaching for the True Power. It isn't possible to shield one person out of a circle since, in effect, the circle has become a single person for the purpose of channeling. You would have to shield the entire circle, which would require either a circle of your own or a pretty hefty sa'angreal. A Warder cannot feel that his Aes Sedai has been shielded, though he would be aware of any agitation on her part. But this would tell him no more than that she was agitated.