code: public void setSelection(List<GraphUser> graphUsers) {
          List<String> userIds = new ArrayList<String>();
          for(GraphUser graphUser: graphUsers) {
              userIds.add(graphUser.getId());
          }
          setSelectionByIds(userIds);
      }
docs: Sets the list of friends for pre selection. These friends will be selected by default.
  @param graphUsers list of friends as GraphUsers