I disagree Greg,
if you have a good vocal blend and one person triggers the threshold, everything in that group gets lowered, not just the offending vocal.
It can not work any other way, regardless of whether or not the vocal mix is out of whack.
Now, if the compression isn't noticeable, it's because the compression settings were so timid that compressing the group was pointless. It is why I never do that.
Each vocalist gets his/her own compression based on their tendencies. If I do not know the vocalists and their tendancies, then each one still gets their own compression, even more important.
No sweat, keep doing this if it works for you.