CurrentItem["Stakeholders"] = usercollection; //Type: SPFieldUserValueCollection
 workflowProperties.Item.SystemUpdate(false); //Exception here
 //workflowProperties.Item.Update();  //Exception here


//CurrentItem Definition
private SPListItem CurrentItem
{
     get
     {
         SPList list = workflowProperties.Web.Lists[workflowProperties.ListId];
         SPListItem li = list.GetItemById(workflowProperties.ItemId);
         return li;
      }
}