Here are some not so obvious and very tactful techniques and guidances I have used to apply the much needed polish to Various Groups. If I might give you a few points:
* Try some blending exercises...have one person begin singing a straight note, have another person come in, singing the same note
without changing the volume, sound or the pitch......I use to do it one by one and tape it so each person could hear themselves.
* Always put the best vocals in row 1 directly in front of the microphone...Always!!!!!
* I know you are probably doing enunciation exercises with the group but try individual enuciating exercises so people can hear themselves and the way they pronounce the words they sing. For instantance AAAAAA sound should be sung with your mouth in the shape of an O....It is best practice to keep your mouth in the shape of an O instead of in the shape of the wide smile because the words often come out sloppy. (Sometimes people will self correct these issues when uou point them out, other times they cannot hear and do not understand) Always add the last letter to your words.....
* I have always told the choirs,groups, praise teams etc.,I have trained if you do not know or if you cannot hit your note...open your mouth (make the motion like you are singing) but don't let anything come out.
*Final thought...there are studio quality vocal people and then there are people who are great group members but are not vocally suited for studio work.
I hope this helps!!!!!!!