$ echo "host1:group1:9001:user1,user2,user3,user4,user5
host1:group2:9002:user1,user2,user3
host1:group3:9003:user1,user2,user4
host1:group4:9004:user1,user2,user5
host1:group5:9005:user1,user2
host1:group6:9006:
host2:group1:9001:user1,user2,user3,user4,user5
host2:group2:9002:user1,user2,user3
host2:group3:9003:user1,user2,user4
host2:group4:9004:user1,user2,user5
host2:group5:9005:user1,user2
host2:group1:9006:
host3:group1:9001:user1,user2,user3,user4,user5
host3:group2:9002:user1,user2,user3
host3:group3:9003:user1,user2,user4
host3:group4:9004:user1,user2,user5
host3:group5:9005:user1,user2
host3:group1:9006:" | perl -e'

use strict;
use warnings;

my %table;
while ( <> ) {
    chomp;
    my ( $host, $grp, $gid, $userid ) = split /:/;
    my $group = "$grp:$gid";
    $table{ $group }{ $userid } = 1;
    }

for my $group ( sort keys %table ) {
    print "$group:";
    my @users = sort keys %{ $table{ $group } };
    print join( ",", @users ), "\n";
    }
'
group1:9001:user1,user2,user3,user4,user5
group1:9006:
group2:9002:user1,user2,user3
group3:9003:user1,user2,user4
group4:9004:user1,user2,user5
group5:9005:user1,user2
group6:9006: