Using Command "Chage" for Everyone in the Group Group

I don't think there's a one-shot command to do this, so you would need to extract the list of members from the groups database and loop over it - exactly how you choose to do that is up to you but for example

#!/bin/bash

group='group'
mindays=28

IFS=, read -a members < <(getent group "$group" | cut -d: -f4)

for logname in "${members[@]}"; do
  echo chage -m "$mindays" "$logname"
done

(remove the echo once you are satisfied that it will do the right thing).