docs: def add(self, obj):
          """ Adds a member to the collection.
  
          :param obj: Object to add.
  
          Example:
              >>> mycollection.add(pump.Person('bob@example.org'))
          """
          activity = {
              "verb": "add",
              "object": {
                  "objectType": obj.object_type,
                  "id": obj.id
              },
              "target": {
                  "objectType": self.object_type,
                  "id": self.id
              }
          }
  
          self._post_activity(activity)
  
          # Remove the cash so it's re-generated next time it's needed
          self._members = None