code: public GroupOptions addGroupByField(Field field) {
  
  		Assert.notNull(field, "Field for grouping must not be null.");
  		Assert.hasText(field.getName(), "Field.name for grouping must not be null/empty.");
  		groupByFields.add(field);
  		return this;
  	}
docs: Adds a group request for a {@link Field}.
  
  @param field
  @return