D365 grid selected row id into javascript

 If you want to get all selected values of a subgrid you can try with this sample.

 var gridContext = formContext.getControl("AttendeesSubgrid");  
     var allSelectedRows = gridContext.getGrid().getSelectedRows();  
     var attendeeList = [];  
     allSelectedRows.forEach(GetSelectedRecord);  
     function GetSelectedRecord(item) {  
       var rowData = item._entityId;  
       var attendiee = rowData.guid;  
       attendeeList.push(attendiee);  
     }