I remember very clearly while teaching kindergarten music one day, I asked the children to sing a phrase back to me somewhat like call and response. The children did as asked with the notable exception of one young man. When I saw that he wasn't participating, I asked what the problem was. He simply looked up at me and asked if they had to make their voice wiggle just like mine. Oops. I knew I needed to smooth out the sound for the young ones.
When I was taking vocal lessons, we never did really cover controlling the "wigglies" or vibrato in the voice. In fact, until an adjudicator told me he like my use of vibrato in a particular piece, I really hadn't ever thought about it.
However, I do remember well, an older lady in our church choir who had magnificent vibrato. Her voice could have loosened plaster (maybe it did). There was so much wiggle in there that it was well nigh impossible to sing beside her and sing in tune. She had been the mainstay of the choir for a very long time and it was really hard for her to rein it in to blend with the rest of the choir. Perhaps, she like me, didn't even know that you could control the "wigglies".
When singing as a member of a group, you really must listen more than sing. Someone said that God gave us two ears and one mouth for a reason. We must hear how our voice blends with others. People with very strong voices have to work hard at that for sure.
We have some awesome choir members. Some read music, some are learning but all working together make a marvellous sound. Once I explain the sound we are looking for, we practice it until it works. It takes a concerted effort and because our choir is smaller, we cannot have voices that "stick out". Cupping your hand behind your ear does help you to be able to hear yourself. Also, because Damon is recording our practices, we can not only have a practice CD but I am able to hear the blend or lack thereof. We continue to work on that.
Listen to some of your favourite groups singing. Why do you like their sound? What makes it work? I would love to hear your ideas.