Folks, I got a working solution

Thanks to 'balajesuri' from 'The UNIX and Linux Forums' for this solution.

#!/usr/bin/perl
use strict;
use warnings;

my ($gid, $grp, $host, $group, $userid);
my %table;

open FILE, "< /path/to/usrgrps.txt";
foreach (<FILE>) {
    chomp;
    ($host, $grp, $gid, $userid) = split /:/;
    $group = "$grp:$gid";
    if (length ($userid) != 0) {
        (defined $table{$group} && $table{$group} =~ /$_/) ? next : ($
+table{$group}.="$_,") foreach (split /,/, $userid);
    }
    else {
        $table{$group} = "";
    }
}
close FILE;

foreach (sort keys %table) {
    $table{$_} =~ s/,$//;
    print "$_:$table{$_}\n";
}
[download]

Thank again to all for taking the time to look at this!!

All the best, g