The add/edit operations are for the groups, but the group contents will just be low level array edits.
Take a look at the attached sample script, I tested it and it's working. Basically the logic is the same, the difference is you edit the group, but just add the vm moref of the vm you want to add to the group by just sending in an updated array of vm morefs.
perl addVm2DrsGroup.pl --server=172.16.254.50 --username=administrator@vlab --password=* --cluster=CLU01 --drsgroup=group1 --vm=recovery-03