To add assignee column for Risk item or Downstream Column (task), use:


},{
      "headerGroup":"Evaluation",
      "headerGroupCss":"headSetRanking",
      "headerCss":"headSetRanking",
        "header": "Im.ID",
        "type": "taskLink",
       "cellCss": "grayCell",
       "id":"task",
       "filterable": true,
        "width": 80
      },{
      "headerGroup":"Evaluation",
      "headerGroupCss":"headSetRanking",
      "headerCss":"headSetRanking",
       "header": "Improvement Title",
         "headerTooltip": "Task Title",
         "bindings": "task.title",
            "id":"taskTitle",
       "filterable": true,
          "minWidth": 150
      },{
           "headerGroup":"Evaluation",
      "headerGroupCss":"headSetRanking",
      "headerCss":"headSetRanking",
       "header": "T. Assignee",
         "headerTooltip": "Task Assignee",
         "bindings": "task.assignee",
         "type":"ref:user",
         "typeProperties": {
               "userRole":"project_assignable"
         },
       "filterable": true,
         "minWidth": 150
        }

see especially


         "bindings": "task.assignee",
         "type":"ref:user",
         "typeProperties": {
               "userRole":"project_assignable"
         },


where bindings can be "assignee" for risk items, or "DOWNSTREAMITEMID.assignee"