code: def register_entity_to_group(self, entity, group):
          '''
          Add entity to a group.
          If group does not exist, entity will be added as first member
          entity is of type Entity
          group is a string that is the name of the group
          '''
          if entity in self._entities:
              if group in self._groups:
                  self._groups[group].append(entity)
              else:
                  self._groups[group] = [entity]
          else:
              raise UnmanagedEntityError(entityf register_entity_to_group(self, entity, group):
          '''
          Add entity to a group.
          If group does not exist, entity will be added as first member
          entity is of type Entity
          group is a string that is the name of the group
          '''
          if entity in self._entities:
              if group in self._groups:
                  self._groups[group].append(entity)
              else:
                  self._groups[group] = [entity]
          else:
              raise UnmanagedEntityError(entity)
docs: Add entity to a group.
          If group does not exist, entity will be added as first member
          entity is of type Entity
          group is a string that is the name of the group