docs: public BoxGroupMembership.Info addMembership(BoxUser user, Role role) {
          BoxAPIConnection api = this.getAPI();
  
          JsonObject requestJSON = new JsonObject();
          requestJSON.add("user", new JsonObject().add("id", user.getID()));
          requestJSON.add("group", new JsonObject().add("id", this.getID()));
          if (role != null) {
              requestJSON.add("role", role.toJSONString());
          }
  
          URL url = ADD_MEMBERSHIP_URL_TEMPLATE.build(api.getBaseURL());
          BoxJSONRequest request = new BoxJSONRequest(api, url, "POST");
          request.setBody(requestJSON.toString());
          BoxJSONResponse response = (BoxJSONResponse) request.send();
          JsonObject responseJSON = JsonObject.readFrom(response.getJSON());
  
          BoxGroupMembership membership = new BoxGroupMembership(api, responseJSON.get("id").asString());
          return membership.new Info(responseJSON);
      }