class Player(models.Model):
    fname = models.CharField(max_length = 50)
    lname = models.CharField(max_length = 50)
    elo = models.IntegerField(max_length = 10)
    def __unicode__(self):
        return '%s %s' % (self.fname, self.lname)

class Party(models.Model):
    players = models.ManyToManyField(Player, related_name = 'pl')
    winner = models.ForeignKey(Player, related_name = 'win')

from django.db.models.signals import post_save
def create_party(sender, instance, created, **kwargs):
    if created:
        players = instance.players.all()
        winner = instance.winner
        #Что-то делаете с players и сохраняете
        for player in players:
            player.fname = .......
            player.lname = .......
            player.elo = .......
            player.save()
        
        #Что-то делаете с winner и сохраняете
        winner.fname = .......
        winner.lname = .......
        winner.elo = .......
        winner.save()
post_save.connect(create_party, sender=Party)