perl -ane '{$t5{$F[3]}+=$F[5]; $t6{$F[3]}+=$F[6]; $t{$F[3]}+=1} 
                  END{foreach $i (keys %t5) 
                          {print "Group $i ",$t5{$i}/$t{$i}," ",$t6{$i}/$t{$i},"\n";}}  ' < dat.txt
Group S -75.969 10.2868
Group G -108.8372 73.3412
Group M -72.0558 7.1884