my %group2user;
foreach my $rec (@all_groups) {
  push @{$group2user{$rec->[0]}}, $rec;
}
foreach my $group_id (sort keys %group2user) {
  # Do whatever
}

##</code><code>##

my $old_id;
for my $rec (@all_groups) {
  if (not defined $old_id or $old_id <=> $rec->[0]) {
    # Spit out header
    $old_id = $rec->[0];
  }
  # Print user record
}