This script will loop through all users and use dseditgroup to check if they are a member of a given group (macOS)

checkmembers.sh

#!/bin/bash

export PATH=/usr/bin:/bin:/usr/sbin:/sbin

# checkusers
#
# written by Armin Briegel, Scripting OS X, 2018
#
# code is provided as is, with no guarantees that it will work etc.
# adapt and use as you will
#
# 
#/ loops through all users to see if they are member of a group
#/
#/ usage:
#/     checkusers.sh groupname [node]
#/         - groupname: short name for the group to list users for
#/         - node: directory node path to search for user
#/                 defaults to /Local/Default
#/ 
#/ Note: this script will loop over _every_ user in the system and use
#/ dseditgroup -o checkmember
#/ to see if the system considers this user a member of the given group
#/ in large directory server deployments this will be very slow and create
#/ a lot of traffic to the directory

usage() {
    grep '^#/' "$0" | cut -c4-
    exit 0
}

# first see if $1 is valid
if [[ -z $1 ]]; then
    echo "needs groupname as argument"
    usage
    exit 1
fi

if ! dscl /Search read /Groups/"$1" 1>/dev/null ; then
    echo "no group named $1"
    usage
    exit 1
fi

node=${2:-"/Local/Default"}

for u in $(dscl "$node" list /Users ); do
    if dseditgroup -o checkmember -m "$u" "$1" 1>/dev/null ; then
        echo "$u"
    fi
done