Why would/do you need to try to replicate what the Approval - Group activity already does?  Why not add a similar script to that activity instead?  When a record is inserted on the sysapproval_group table the SNC - Create user approvals for group Business Rule calls a Script Include to create the individual approvals, so you could take a look at that to see if there's a relation back to the group that is not getting set with your script.