def add_to_groups(self, username, groups):
        """ Add a user to a list of groups

        :param username: 'key' attribute of the user
        :type username: string
        :param groups: list of groups
        :type groups: list of strings
        """
        self._check_fix_users(username)
        current_groups = self.users[username]['groups']
        new_groups = current_groups | set(groups)
        self.users[username]['groups'] = new_groups
Add a user to a list of groups

        :param username: 'key' attribute of the user
        :type username: string
        :param groups: list of groups
        :type groups: list of strings