Hi SandorV
Current code which you have used is for assigning workflow to a user.To assign it to a group use following
workflow.parameters["bpm:groupAssignee"] = people.getGroup("GROUP_MyGroupName");
However make sure that the workflow which you are trigerring is a group workflow. OOTB group review workflows is"activitiParallelGroupReview" for parallel review and for pooled review [seen in Start Workflow screen as "Parallel Group Review And Approve Activiti Process"]
Regards