Are they used to sing with mics?
Do they use mics live?
Do they have good mic technique?
You've missed the point.
A barbershop
quartet, or indeed any bunch of musicians that have their own internal
balance have no need, or desire to have any mic technique at all. They blend their voices so that once a few feet away, their voices combine into 'a whole'. So stereo mic techniques are needed, so they never get close enough to even touch a mic. The job of the microphones is to capture what is there, not capture individuals, just their contribution to the 'whole'.
Mic technique, as applied to vocalists, using the mic as part of their sound, is simply not a feature.
When people who are used to using mics sing barbershop style, like the Osmonds, for example, have hand held mics - they are professional enough to ensure the mic technique they employ allows the sound op to blend them properly. This technique is really prone to going wrong badly if the singers are not really 100% solid with their
performance and mic technique - it shouldn't ever be applied to anyone other than those who really, really understand it!