For those of you in, or near to, Edinburgh, the Scots Music Group run a fantastic harmony singing class lead by Yvonne Burgess. Everything done by ear, excellent warm-ups, and a real emphasis on listening to what the other voices are doing. I stayed in the class for several years, even after 'learning' to sing harmony, for the sheer joy of it - the group makes such a beautiful noise!

Otherwise I very much agree with WyoWoman's comments - if you can hear you can harmonize.