Please, just use perl already. Here's something to get started. At least it reliably parses the file without the sed grep nonsense.
#!/usr/bin/perl -w
use strict;
my $fp;
open $fp, "/etc/group" or die "poop";
while (<$fp>) {
chomp;
my ($gname, $gpass, $gid, $dudes) = split /:/;
my @users = split /,/, $dudes;
print "$gname has @users in it\n";
}
close $fp;
