<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def add_user(self, ):
 """Add a user and store it in the self.users :returns: None :rtype: None :raises: None """

        i = self.user_tablev.currentIndex()
        item = i.internalPointer()
        if item:
            user = item.internal_data()
            if self._project:
                self._project.users.add(user)
            else:
                self._task.users.add(user)
            self.users.append(user)
            item.set_parent(None)