domain1 NS ns1
domain1 NS ns2
domain1 NS ns3
domain2 NS dnsx

domain1,ns1,ns2,ns3
domain2, dnsx

awk '
    $1 != domain {
        if (domain) print ""
        printf "%s", $1
        domain = $1
    }
    {printf ",%s" $3}
    END {print ""}
' file