apt install pssh
parallel-ssh -h HOSTNAMES_TXT -o /tmp/users 'cut -d: -f1 /etc/passwd'
perl -le '
    for my $file (@ARGV) {
        my ($host) = $file =~ m{/([^/]+)$};
        open my($fh), $file
           or die "Error opening $file: $!";
        my @users = <$fh>;
        chomp @users;
        print join(",", $host, @users) . "\n";
    }
' /tmp/users/*