How to assign a workflow to usergroup in AEM 6.3 using osgi R6 annotations?

Try the below code. Make sure you have the R6 annotations dependencies in the pom.

import com.day.cq.workflow.WorkflowException;

import com.day.cq.workflow.WorkflowSession;

import com.day.cq.workflow.exec.ParticipantStepChooser;

import com.day.cq.workflow.exec.WorkItem;

import com.day.cq.workflow.metadata.MetaDataMap;

import org.osgi.service.component.annotations.Component;

@Component(service = ParticipantStepChooser.class, property = {"chooser.label = A Test Particpant Chooser step"})

public class DynamicParticipantStepChooser implements ParticipantStepChooser {

   @Override
   public String getParticipant(WorkItem workItem, WorkflowSession workflowSession, MetaDataMap metaDataMap) throws WorkflowException {

   // code
   return "administrators";

  }

}