code: func addContactsData(contactIDs []string) string {
  	cap := len(contactIDs)
  	params := make([]string, 0, cap)
  
  	for _, contactID := range contactIDs {
  		params = append(params, "ids[]="+contactID)
  	}
  
  	return strings.Join(params, "&")
  }
docs: // addContactsData gets the data string for adding a contact to a group. We're
  // intentionally not using url.Values for building the string: the API expects
  // `ids[]=foo&ids[]=bar` format, while url.Values encodes to `ids=foo&ids=bar`.