code: func AddContacts(c *messagebird.Client, groupID string, contactIDs []string) error {
  	if err := validateAddContacts(contactIDs); err != nil {
  		return err
  	}
  
  	data := addContactsData(contactIDs)
  
  	return c.Request(nil, http.MethodPut, path+"/"+groupID+"/"+contactPath, data)
  }
docs: // AddContacts adds a maximum of 50 contacts to the group.