Adds a member of an authorization group.

def auth_add(name, service, **kwargs):
    """
    Adds a member of an authorization group.
    """
    ctx = Context(**kwargs)
    ctx.execute_action('auth:group:members:add', **{
        'storage': ctx.repo.create_secure_service('storage'),
        'name': name,
        'service': service,
    })