local $/ = '';
                                                                                
while (<DATA>)
{
  chomp;
                                                                                
  my( $group, $number, $members) =  split /,/,$_,3;
                                                                                
  @members = ($members =~ /(CN=[^,]+)/mg);
                                                                                
  $members = '"' . join( ',', @members) . '"';
                                                                                
  print "$group,$number,$members\n";
                                                                                
}
__DATA__
...