Snowy's Mom, that sounds like the problem that I have with Daisy and Perry (both Schnauzers) They like certain people, one Grandson, they love kisses and all, the other two they bark at and go crazy. I think it's when the two were younger they would scream and jump around and got the dogs excited. If anyone comes around that they don't know, they bark and bark. I think Carrie and Souraya's advice is excellent.
We take them out riding quite often to get them used to seeing people and dogs and they are doing better. Good luck, as Carrie says if you have a Schnauzer or most likely any Terrier you either love them or leave them.