Some styles of music demand straighter tone, and in any case you need to match vibratos with the other singers. Good singing is not just about melody, but rhythm is equally important. Again, that music fundamentals class may help. But you might also get together with a drummer and have he or she play and tape various beats. When you sing any style, you want to be aware of the basic pulse of the groove and of the subdivisions, such as eighth or sixteenth notes.
Syncopations lock up with the subdivisions.
All singers in the group have to lock in with the groove together. A Cappella groups create the whole groove on their own. And again, listen, listen, listen. Listen to Ella swing, to Take Six funk, to Vocal Sampling's salsa. Good harmony singers need to sing the nuances of style as one voice.