class Trip(models.Model):
    members = models.ManyToManyField(User, blank=True, null=True,
        related_name='user_trips', through='TripReservation')


class TripReservation(models.Model):
    user = models.ForeignKey(User)
trip = models.ForeignKey(Trip)
Django — обратный поиск с помощью ManyToManyField