Susan, I am sorry that I haven't replied before now. I spent the past two weeks in the hospital and Mudcat was not, I blush to say, the first thing on my mind. I have a few comments and suggestions for the teaching of harmony singing.
First, harmony is nice but blending is nicer. In the best of all possible ensembles, the voice should be that of the group, rather than the loudest, individual singer. Blending is, surprisingly, easy to achieve. It, just, takes listening. I tell my workshop participants to pretend that there is a microphone in the middle of the room and they should aim their voices at it. Then I have them listen, while they are singing, and, if they can hear their voice clearly, lower their volume and if they can't hear their voice, crank it up until they can. Even unison singing sounds magnificent when the voices are in concert and the most approval hungry singer will be assuaged by the glorious sound of the group. If/when we, ever, get together, I'll show you how easy this is to accomplish.
I will be doing workshops for the Philadelphia Folksong Society this spring. I will let you know where and when. I conduct harmony workshops for their Sping Thing and Fall Fling weekends.
I am sure that you are, or will be, a wonderful song leader. You have the right attitude and the right aptitude.
I suggest that you contact me directly at musicmic@peoplepc.com
We'll talk.

                         Mike