unless there is enforcement to prohibit people who are trying to board out of order, then nothing will matter. but here's an idea i just thought of, assuming they would enforce the numbers.
what if instead of group numbers, you had an actual boarding number (like your seat assignment).
then you could do it like when they call your turn at the deli counter. boarding passengers 1 -20
wait a few minutes, then boarding passengers 21-40
people who want to board together could request a change on the app or with a gate agent, and there'd be some surplus of boarding numbers that could be rearranged without changing the numbers for all the passengers who've been assigned already.
you have
#20 and want to board with
#45 , the app or GA moves you to
#46 which was an unused number so
#47 still boards when
#47 is called.
changes are handled like the seating arrangements when people ask if the gate agents if they can be moved to be seated together.
dunno, just a random thought. like i said, without enforcement and prohibition, nothing matters.