#!/usr/bin/perl -w
use strict;

open my $fh_group, "</etc/group" or die "Cannot open group file";

while (<$fh_group>) {
  chomp;
  my ($grp, $pwd, $gid, $users) = split /:/;
  if ($grp eq $ARGV[0]) {
    my @users = split /,/, $users;
    print join $/, @users;
  }
}