I have a degree of instinctual ability for improvising harmonies, developed since early childhood when my mom used to harmonize with us kids, but sometimes I get overconfident and find myself singing a real clunker of a note, or (at other times) unable to come up with anything other than just singing in unison with the lead part.

Given the opportunity to rehearse a given piece over and over, and to work up a vocal arrangement, I'd probably be able to cover pretty much any angle. But the point is, what I can do is semi-unconscious, and therefore virtually impossible to pass along to someone else.

I have long since despaired of understanding the different "learning styles" and musicial difficulties of different people. I've encountered too many folks over the years who are far more accomplished than I am as instrumentalists and/or as singers, but who are flat-out unable to do one or two particular things that come easily to me.

Perhaps this is an oversimplification, but I feel that musical ability consists of a combination of instinctive "feeling" and studied, almost mechanical, training, and that everyone needs a bit of both to participate in music-making at all. But everyone seems to strike a different balance between the two extremes, and folks who are too far apart from each other along this "spectrum" can have great difficulty with musical communication.

It's a good thing so many different suggestions have been offered here. Whatever MHCF's problem might be, one approach might work for him while another won't. Just try 'em all, I guess, and keep at it. Assuming that all three members of the group are sufficiently committed, this is a problem that will probably be solved before too much longer.