Add a user and store it in the self.users

        :returns: None
        :rtype: None
        :raises: None

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)