My friend who used to photograph weddings once wondered why his flash couldn't keep up at f5.6 when inside. I have enough troubles at f2.8.
so how do those group shots turn out at f2.8
I regularly shoot f4 to f8 indoors at night at weddings with flash, 2nd curtain sync slow shutter speed to keep the iso reasonable. job done.
Its pretty straight forward really.
To be honest if your friend was wondering that, then I'm wondering what he was doing shooting weddings in the first place.
Also 100% agree with bdunbar ETTR is critical at higher isos make sure your highlight alert is on and shoot as far over exposed without blowing out skin then reduce the exposure in LR dial in some shadow recovery and apply NR typically i'll over expose from the recomended meter settings but 2/3 to 1 and 1/3 stops usually set at 1 stop over and tweek as needed, I always shoot in manual as Av or anything auto will balls this up.